[Skip to Navigation]
Sign In
Figure 1.  Study Flow Diagram
Study Flow Diagram
Figure 2.  Forest Plot for Studies of Incubation Period of COVID-19 Caused by Different Variants
Forest Plot for Studies of Incubation Period of COVID-19 Caused by Different Variants
Figure 3.  Incubation Period for COVID-19 in Older Patients and Infected Children
Incubation Period for COVID-19 in Older Patients and Infected Children

Infectious strains were original strains.

Figure 4.  Incubation Period for COVID-19 in Patients With Severe and Nonsevere Illness
Incubation Period for COVID-19 in Patients With Severe and Nonsevere Illness

Infectious strains were original strains.

Table.  Mean Incubation Period of COVID-19 From Included Studies
Mean Incubation Period of COVID-19 From Included Studies
1.
Guan  W-J, Ni  Z-Y, Hu  Y,  et al; China Medical Treatment Expert Group for Covid-19.  Clinical characteristics of coronavirus disease 2019 in China.   N Engl J Med. 2020;382(18):1708-1720. doi:10.1056/NEJMoa2002032PubMedGoogle ScholarCrossref
2.
World Health Organization. Coronavirus (COVID-19) Dashboard. 2022; https://covid19.who.int/.
3.
Sanders  JM, Monogue  ML, Jodlowski  TZ, Cutrell  JB.  Pharmacologic treatments for coronavirus disease 2019 (COVID-19): a review.   JAMA. 2020;323(18):1824-1836. doi:10.1001/jama.2020.6019PubMedGoogle ScholarCrossref
4.
Gostic  K, Gomez  AC, Mummah  RO, Kucharski  AJ, Lloyd-Smith  JO.  Estimated effectiveness of symptom and risk screening to prevent the spread of COVID-19.   Elife. 2020;9:9. doi:10.7554/eLife.55570PubMedGoogle ScholarCrossref
5.
Tawe Ngi  AM, Johnston  S, Albayat  SS,  et al.  Pre-symptomatic and asymptomatic transmission of COVID-19: Implications for control measures in Qatar.   Qatar Med J. 2021;2021(3):59-59.PubMedGoogle Scholar
6.
Ganyani  T, Kremer  C, Chen  D,  et al.  Estimating the generation interval for coronavirus disease (COVID-19) based on symptom onset data, March 2020.   Euro Surveill. 2020;25(17):2000257. doi:10.2807/1560-7917.ES.2020.25.17.2000257Google ScholarCrossref
7.
Nishiura  H, Mizumoto  K, Ejima  K, Zhong  Y, Cowling  B, Omori  R.  Incubation period as part of the case definition of severe respiratory illness caused by a novel coronavirus.   Euro Surveill. 2012;17(42):20296. PubMedGoogle ScholarCrossref
8.
Nishiura  H, Linton  NM, Akhmetzhanov  AR.  Serial interval of novel coronavirus (COVID-19) infections.   Int J Infect Dis. 2020;93:284-286. doi:10.1016/j.ijid.2020.02.060Google ScholarCrossref
9.
World Health Organization. Tracking SARS-CoV-2 variants. Updated July 19, 2022. Accessed April 17, 2022. https://www.who.int/en/activities/tracking-SARS-CoV-2-variants/
10.
US Centers for Disease Control and Prevention. SARS-CoV-2 Variant Classifications and Definitions. Updated April 26, 2021. Accessed April 17, 2022. https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-classifications.html#anchor_1632158885160
11.
Wan  X, Wang  W, Liu  J, Tong  T.  Estimating the sample mean and standard deviation from the sample size, median, range and/or interquartile range.   BMC Med Res Methodol. 2014;14:135. doi:10.1186/1471-2288-14-135PubMedGoogle ScholarCrossref
12.
Hozo  SP, Djulbegovic  B, Hozo  I.  Estimating the mean and variance from the median, range, and the size of a sample.   BMC Med Res Methodol. 2005;5:13. doi:10.1186/1471-2288-5-13PubMedGoogle ScholarCrossref
13.
Wells  G, Shea  B, O’Connell  D,  et al. The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analyses. Published 2021. Accessed April 17, 2022. http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp
14.
McAloon  C, Collins  Á, Hunt  K,  et al.  Incubation period of COVID-19: a rapid systematic review and meta-analysis of observational research.   BMJ Open. 2020;10(8):e039652-e039652. doi:10.1136/bmjopen-2020-039652PubMedGoogle ScholarCrossref
15.
Melsen  WG, Bootsma  MC, Rovers  MM, Bonten  MJ.  The effects of clinical and statistical heterogeneity on the predictive values of results from meta-analyses.   Clin Microbiol Infect. 2014;20(2):123-129. doi:10.1111/1469-0691.12494Google ScholarCrossref
16.
Areekal  B, Vijayan  SM, Suseela  MS.  Risk factors, epidemiological and clinical outcome of close contacts of COVID-19 cases in a tertiary hospital in southern India.  J Clin Diagn Res. 2021;15(3):LC34-LC37.
17.
Backer  JA, Eggink  D, Andeweg  SP,  et al.  Shorter serial intervals in SARS-CoV-2 cases with Omicron BA.1 variant compared with Delta variant, the Netherlands, 13 to 26 December 2021.   Euro Surveill. 2022;27(6):2200042. doi:10.2807/1560-7917.ES.2022.27.6.2200042Google ScholarCrossref
18.
Backer  JA, Klinkenberg  D, Wallinga  J.  Incubation period of 2019 novel coronavirus (2019-nCoV) infections among travelers from Wuhan, China, 20-28 January 2020.   Euro Surveill. 2020;25(5):2000062. doi:10.2807/1560-7917.ES.2020.25.5.2000062Google ScholarCrossref
19.
Bao  C, Pan  E, Ai  J,  et al.  COVID-19 outbreak following a single patient exposure at an entertainment site: an epidemiological study.   Transbound Emerg Dis. 2021;68(2):773-781. doi:10.1111/tbed.13742PubMedGoogle ScholarCrossref
20.
Brandal  LT, MacDonald  E, Veneti  L,  et al.  Outbreak caused by the SARS-CoV-2 Omicron variant in Norway, November to December 2021.   Euro Surveill. 2021;26(50):2101147. doi:10.2807/1560-7917.ES.2021.26.50.2101147Google ScholarCrossref
21.
Bui  LV, Nguyen  HT, Levine  H,  et al.  Estimation of the incubation period of COVID-19 in Vietnam.   PLoS One. 2020;15(12):e0243889-e0243889. doi:10.1371/journal.pone.0243889PubMedGoogle ScholarCrossref
22.
Chen  G, Wu  MZ, Qin  CJ,  et al.  Epidemiological analysis of 18 patients with COVID-19.   Eur Rev Med Pharmacol Sci. 2020;24(23):12522-12526.PubMedGoogle Scholar
23.
Zhang  W; Covid-Epidemiology Investigation Team; Laboratory Testing Team.  Local outbreak of COVID-19 in Shunyi District attributed to an asymptomatic carrier with a history of stay in Indonesia—Beijing municipality, China, December 23, 2020.   China CDC Wkly. 2021;3(10):214-217. doi:10.46234/ccdcw2020.062PubMedGoogle ScholarCrossref
24.
Dai  J, Yang  L, Zhao  J.  Probable longer incubation period for elderly COVID-19 cases: analysis of 180 contact tracing data in Hubei Province, China.   Risk Manag Healthc Policy. 2020;13:1111-1117. doi:10.2147/RMHP.S257907PubMedGoogle ScholarCrossref
25.
de Laval  F, Grosset-Janin  A, Delon  F,  et al.  Lessons learned from the investigation of a COVID-19 cluster in Creil, France: effectiveness of targeting symptomatic cases and conducting contact tracing around them.   BMC Infect Dis. 2021;21(1):457-457. doi:10.1186/s12879-021-06166-9PubMedGoogle ScholarCrossref
26.
Del Águila-Mejía  J, Wallmann  R, Calvo-Montes  J, Rodríguez-Lozano  J, Valle-Madrazo  T, Aginagalde-Llorente  A.  Secondary attack rate, transmission and incubation periods, and serial interval of SARS-CoV-2 Omicron variant, Spain.   Emerg Infect Dis. 2022;28(6):1224-1228. doi:10.3201/eid2806.220158PubMedGoogle ScholarCrossref
27.
Deng  L, Li  Z, Liu  Y,  et al.  Epidemiological and clinical findings of discharge patients infected with the 2019 novel coronavirus (SARS-CoV-2) in Changchun, Northeast China: a retrospective cohort study.  Acta Med Mediter. 2021;37(2):1147-1153.
28.
Deng  Y, You  C, Liu  Y, Qin  J, Zhou  X-H.  Estimation of incubation period and generation time based on observed length-biased epidemic cohort with censoring for COVID-19 outbreak in China.   Biometrics. 2021;77(3):929-941. doi:10.1111/biom.13325PubMedGoogle ScholarCrossref
29.
Denis  F, Septans  AL, Le Goff  F, Jeanneau  S, Lescure  FX.  Analysis of COVID-19 transmission sources in France by self-assessment before and after the partial lockdown: observational study.   J Med Internet Res. 2021;23(5):e26932. doi:10.2196/26932PubMedGoogle ScholarCrossref
30.
Ding  K, Yi  B, Chen  Y,  et al.  Epidemic cluster of novel coronavirus disease 2019 in Ningbo city of Zhejiang province.  Article in Chinese.  Chin J Publ Health. 2020;36(04):498-502.Google Scholar
31.
Dong  XC, Li  JM, Bai  JY,  et al.  [Epidemiological characteristics of confirmed COVID-19 cases in Tianjin].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(5):638-641.PubMedGoogle Scholar
32.
Du  ZC, Gu  J, Li  JH,  et al.  [Estimating the distribution of COVID-19 incubation period by interval-censored data estimation method].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(7):1000-1003.PubMedGoogle Scholar
33.
Gao  Y, Hou  L, Li  Y,  et al.  Epidemiological investigation and analysis of 137 confirmed cases of COVID-19.  Article in Chinese.  Chinese Journal of Critical Care Medicine. 2020;40(03):232-236.Google Scholar
34.
Gao  Y, Ma  X, Bi  J,  et al.  Epidemiological and clinical differences of coronavirus disease 2019 patients with distinct viral exposure history.   Virulence. 2020;11(1):1015-1023. doi:10.1080/21505594.2020.1802870PubMedGoogle ScholarCrossref
35.
Grant  R, Charmet  T, Schaeffer  L,  et al.  Impact of SARS-CoV-2 Delta variant on incubation, transmission settings and vaccine effectiveness: results from a nationwide case-control study in France.   Lancet Reg Health Eur. 2022;13:100278-100278. doi:10.1016/j.lanepe.2021.100278PubMedGoogle ScholarCrossref
36.
Guo  C-X, He  L, Yin  J-Y,  et al.  Epidemiological and clinical features of pediatric COVID-19.   BMC Med. 2020;18(1):250-250. doi:10.1186/s12916-020-01719-2PubMedGoogle ScholarCrossref
37.
Han  T.  Outbreak investigation: transmission of COVID-19 started from a spa facility in a local community in Korea.   Epidemiol Health. 2020;42:e2020056-e2020056. doi:10.4178/epih.e2020056PubMedGoogle ScholarCrossref
38.
Han  T, Hua  L, He  S,  et al.  The epidemiological characteristics of cluster transmission of coronavirus disease 2019 (COVID-19): a multi-center study in Jiangsu Province.   Am J Transl Res. 2020;12(10):6434-6444.PubMedGoogle Scholar
39.
Han  YN, Feng  ZW, Sun  LN,  et al.  A comparative-descriptive analysis of clinical characteristics in 2019-coronavirus-infected children and adults.   J Med Virol. 2020;92(9):1596-1602. doi:10.1002/jmv.25835PubMedGoogle ScholarCrossref
40.
Hong  H, Shi  HB, Jiang  HB,  et al.  [Epidemic dynamic model based evaluation of effectiveness of prevention and control strategies for COVID-19 in Ningbo].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(10):1606-1610.PubMedGoogle Scholar
41.
Hua  C-Z, Miao  Z-P, Zheng  J-S,  et al.  Epidemiological features and viral shedding in children with SARS-CoV-2 infection.   J Med Virol. 2020;92(11):2804-2812. doi:10.1002/jmv.26180PubMedGoogle Scholar
42.
Huang  J, Zhao  Q, Xu  K,  et al.  Analysis of COVID-19 epidemic characteristics based on network data.  Article in Chinese.  Chinese Journal of Disease Control & Prevention. 2020;24(11):1338.Google Scholar
43.
Huang  L, Zhang  X, Xu  A.  Effectiveness of interventions as part of the One Health approach to control coronavirus disease 2019 and stratified case features in Anhui Province, China: a real-world population-based cohort study.   One Health. 2021;12:100224-100224. doi:10.1016/j.onehlt.2021.100224PubMedGoogle Scholar
44.
Huang  S, Li  J, Dai  C,  et al.  Incubation period of coronavirus disease 2019: new implications for intervention and control.   Int J Environ Health Res. 2021;32(8):1707-1715.PubMedGoogle Scholar
45.
Je  D, O’Brolchain  A, Ulett  KB,  et al.  Demographics, clinical characteristics and outcomes among 197 patients with COVID-19 in the Gold Coast area.   Intern Med J. 2021;51(5):666-672. doi:10.1111/imj.15260PubMedGoogle Scholar
46.
Jeong  TH, Pak  C, Ock  M, Lee  SH, Son  JS, Jeon  YJ.  Real asymptomatic SARS-CoV-2 infection might be rare: importance of careful interviews and follow-up.   J Korean Med Sci. 2020;35(37):e333-e333. doi:10.3346/jkms.2020.35.e333PubMedGoogle Scholar
47.
Jiang  G, Wang  C, Song  L,  et al.  Aerosol transmission, an indispensable route of COVID-19 spread: case study of a department-store cluster.   Front Environ Sci Eng. 2021;15(3):46. doi:10.1007/s11783-021-1386-6PubMedGoogle Scholar
48.
Jiang  Z, Yang  B, Qin  J, Zhou  Y.  Enhanced empirical likelihood estimation of incubation period of COVID-19 by integrating published information.   Stat Med. 2021;40(19):4252-4268. doi:10.1002/sim.9026PubMedGoogle Scholar
49.
Jin  X, Lian  J-S, Hu  J-H,  et al.  Epidemiological, clinical and virological characteristics of 74 cases of coronavirus-infected disease 2019 (COVID-19) with gastrointestinal symptoms.   Gut. 2020;69(6):1002-1009. doi:10.1136/gutjnl-2020-320926PubMedGoogle Scholar
50.
Khonyongwa  K, Taori  SK, Soares  A,  et al.  Incidence and outcomes of healthcare-associated COVID-19 infections: significance of delayed diagnosis and correlation with staff absence.   J Hosp Infect. 2020;106(4):663-672. doi:10.1016/j.jhin.2020.10.006PubMedGoogle Scholar
51.
Ki  HJ, Kim  J, Kim  S, Park  J, Lee  J, Kim  Y-J.  Statistical analysis of estimating incubation period distribution and case fatality rate of COVID-19.   Korean Journal of Applied Statistics. 2020;33(6):777-789.Google Scholar
52.
Kim  SE, Jeong  HS, Yu  Y,  et al.  Viral kinetics of SARS-CoV-2 in asymptomatic carriers and presymptomatic patients.   Int J Infect Dis. 2020;95:441-443. doi:10.1016/j.ijid.2020.04.083Google Scholar
53.
Kong  D, Zheng  Y, Wu  H,  et al.  Pre-symptomatic transmission of novel coronavirus in community settings.   Influenza Other Respir Viruses. 2020;14(6):610-614. doi:10.1111/irv.12773PubMedGoogle Scholar
54.
Kong  T-K.  Longer incubation period of coronavirus disease 2019 (COVID-19) in older adults.   Aging Med (Milton). 2020;3(2):102-109. doi:10.1002/agm2.12114PubMedGoogle Scholar
55.
Kong  W, Wang  Y, Hu  J, Chughtai  A, Pu  H; Clinical Research Collaborative Group of Sichuan Provincial People’s Hospital.  Comparison of clinical and epidemiological characteristics of asymptomatic and symptomatic SARS-CoV-2 infection: a multi-center study in Sichuan Province, China.   Travel Med Infect Dis. 2020;37:101754-101754. doi:10.1016/j.tmaid.2020.101754PubMedGoogle Scholar
56.
Lai  C, Yu  R, Wang  M,  et al.  Shorter incubation period is associated with severe disease progression in patients with COVID-19.   Virulence. 2020;11(1):1443-1452. doi:10.1080/21505594.2020.1836894PubMedGoogle Scholar
57.
Lau  YC, Tsang  TK, Kennedy-Shaffer  L,  et al.  Joint estimation of generation time and incubation period for coronavirus disease (Covid-19).   J Infect Dis. 2021;224(10):1664-1671. doi:10.1093/infdis/jiab424PubMedGoogle Scholar
58.
Le  TQM, Takemura  T, Moi  ML,  et al.  Severe acute respiratory syndrome coronavirus 2 shedding by travelers, Vietnam, 2020.   Emerg Infect Dis. 2020;26(7):1624-1626. doi:10.3201/eid2607.200591PubMedGoogle Scholar
59.
Lee  JJ, Choe  YJ, Jeong  H,  et al.  Importation and transmission of SARS-CoV-2 B.1.1.529 (Omicron) variant of concern in Korea, November 2021.   J Korean Med Sci. 2021;36(50):e346-e346. doi:10.3346/jkms.2021.36.e346PubMedGoogle Scholar
60.
Lei  D, Wang  C, Li  C,  et al.  Clinical characteristics of COVID-19 in pregnancy: analysis of nine cases.  Article in Chinese. Chinese Journal of Perinatal Medicine. 2020;12:222-228.
61.
Leung  C.  The difference in the incubation period of 2019 novel coronavirus (SARS-CoV-2) infection between travelers to Hubei and nontravelers: the need for a longer quarantine period.   Infect Control Hosp Epidemiol. 2020;41(5):594-596. doi:10.1017/ice.2020.81PubMedGoogle Scholar
62.
Li  J, Ding  J, Chen  L,  et al.  Epidemiological and clinical characteristics of three family clusters of COVID-19 transmitted by latent patients in China.   Epidemiol Infect. 2020;148:e137-e137. doi:10.1017/S0950268820001491PubMedGoogle Scholar
63.
Li  L, Han  Z-G, Qin  P-Z,  et al.  Transmission and containment of the SARS-CoV-2 Delta variant of concern in Guangzhou, China: a population-based study.   PLoS Negl Trop Dis. 2022;16(1):e0010048-e0010048. doi:10.1371/journal.pntd.0010048PubMedGoogle Scholar
64.
Li  Q, Guan  X, Wu  P,  et al.  Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia.   N Engl J Med. 2020;382(13):1199-1207. doi:10.1056/NEJMoa2001316PubMedGoogle Scholar
65.
Linton  NM, Kobayashi  T, Yang  Y,  et al.  Incubation period and other epidemiological characteristics of 2019 novel coronavirus infections with right truncation: a statistical analysis of publicly available case data.   J Clin Med. 2020;9(2):538. doi:10.3390/jcm9020538PubMedGoogle Scholar
66.
Liu  C-X, Liu  Z-H, Sun  L-Y, Zhang  K-Y, Sun  Y-Z.  A familial cluster of COVID-19 infection in a northern Chinese region.   J Infect Public Health. 2021;14(9):1127-1132. doi:10.1016/j.jiph.2021.07.011PubMedGoogle Scholar
67.
Liu  F, Ji  C, Luo  J,  et al.  Clinical characteristics and corticosteroids application of different clinical types in patients with corona virus disease 2019.   Sci Rep. 2020;10(1):13689-13689. doi:10.1038/s41598-020-70387-2PubMedGoogle Scholar
68.
Liu  J, Liao  X, Qian  S,  et al.  Community transmission of severe acute respiratory syndrome coronavirus 2, Shenzhen, China, 2020.   Emerg Infect Dis. 2020;26(6):1320-1323. doi:10.3201/eid2606.200239PubMedGoogle Scholar
69.
Liu  JY, Chen  TJ, Hwang  SJ.  Analysis of community-acquired COVID-19 cases in Taiwan.   J Chin Med Assoc. 2020;83(12):1087-1092. doi:10.1097/JCMA.0000000000000411PubMedGoogle Scholar
70.
Liu  L, Du  Y, Bai  J,  et al.  Analysis of early characteristics of patients with novel coronavirus infection in Kunming City, Yunnan Province, China.  Article in Chinese. Chinese Journal of Zoonoses. 2020;36(5):424-428.
71.
Liu  L, Jing  L, Li  Y,  et al.  Clustering of 2019 novel coronavirus disease cases in Liaoning province: reported data-base analysis.  Article in Chinese.  Chin J Publ Health. 2020;36(04):473-476. doi:10.11847/zgggws1128823Google Scholar
72.
Liu  P, Niu  R, Chen  J,  et al.  Epidemiological and clinical features in patients with coronavirus disease 2019 outside of Wuhan, China: special focus in asymptomatic patients.   PLoS Negl Trop Dis. 2021;15(3):e0009248-e0009248. doi:10.1371/journal.pntd.0009248PubMedGoogle Scholar
73.
Liu  W, Zhou  M, Yang  S,  et al.  Epidemiological characteristics of cluster cases of coronavirus disease 2019 (COVID-19) in China.   Acta Medicinae Universitatis Scientiae et Technologiae Huazhong. 2020;49(02):161-168.Google Scholar
74.
Liu  Y, Ding  N, Zhou  S,  et al.  Comparison of clinical characteristics between patients with coronavirus disease 2019 (COVID-19) who retested RT-PCR positive versus negative: a retrospective study of data from Nanjing.   J Thorac Dis. 2020;12(11):6435-6445. doi:10.21037/jtd.2020.04.17PubMedGoogle Scholar
75.
Llaque-Quiroz  P, Prudencio-Gamio  R, Echevarría-Lopez  S, Ccorahua-Paz  M, Ugas-Charcape  C.  Clinical and epidemiological characteristics of children with COVID-19 in a pediatric hospital in Peru.   Rev Peru Med Exp Salud Publica. 2020;37(4):689-693. doi:10.17843/rpmesp.2020.374.6198PubMedGoogle Scholar
76.
Mao  S, Huang  T, Yuan  H,  et al.  Epidemiological analysis of 67 local COVID-19 clusters in Sichuan Province, China.   BMC Public Health. 2020;20(1):1525-1525. doi:10.1186/s12889-020-09606-4PubMedGoogle Scholar
77.
Moazzami  B, Moezedin Javad  R, Samie  S,  et al.  Is Computed tomography necessary for the diagnosis of coronavirus disease (COVID-19) in all suspected patients? a case series.  J Kerman Univ Med Sci. 2021;2:187-193.
78.
Ng  T-C, Cheng  H-Y, Chang  H-H,  et al.  Comparison of estimated effectiveness of case-based and population-based interventions on COVID-19 containment in Taiwan.   JAMA Intern Med. 2021;181(7):913-921. doi:10.1001/jamainternmed.2021.1644PubMedGoogle Scholar
79.
Nie  X, Fan  L, Mu  G,  et al.  Epidemiological characteristics and incubation period of 7015 confirmed cases with coronavirus disease 2019 outside Hubei Province in China.   J Infect Dis. 2020;222(1):26-33. doi:10.1093/infdis/jiaa211PubMedGoogle Scholar
80.
Ogata  T, Tanaka  H, Irie  F, Hirayama  A, Takahashi  Y.  Shorter incubation period among unvaccinated delta variant coronavirus disease 2019 patients in Japan.   Int J Environ Res Public Health. 2022;19(3):1127. doi:10.3390/ijerph19031127PubMedGoogle Scholar
81.
Pak  D, Langohr  K, Ning  J, Cortés Martínez  J, Gómez Melis  G, Shen  Y.  Modeling the coronavirus disease 2019 incubation period: impact on quarantine policy.   Mathematics. 2020;8(9):1631. doi:10.3390/math8091631Google Scholar
82.
Pan  XX, Chen  Y, Wang  AH,  et al.  [Study on transmission dynamic of 15 clusters of COVID-2019 cases in Ningbo].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(12):2010-2014.PubMedGoogle Scholar
83.
Patrikar  SR, Kotwal  A, Bhatti  VK,  et al.  Incubation period and reproduction number for novel coronavirus 2019 (COVID-19) infections in India.   Asia Pac J Public Health. 2020;32(8):458-460. doi:10.1177/1010539520956427PubMedGoogle Scholar
84.
Paul  S, Lorin  E.  Distribution of incubation periods of COVID-19 in the Canadian context.   Sci Rep. 2021;11(1):12569-12569. doi:10.1038/s41598-021-91834-8PubMedGoogle Scholar
85.
Ping  K, Lei  M, Gou  Y,  et al.  Epidemiologic characteristics of COVID-19 in Guizhou Province, China.   J Infect Dev Ctries. 2021;15(3):389-397. doi:10.3855/jidc.12818PubMedGoogle Scholar
86.
Pongpirul  WA, Wiboonchutikul  S, Charoenpong  L,  et al.  Clinical course and potential predictive factors for pneumonia of adult patients with Coronavirus Disease 2019 (COVID-19): a retrospective observational analysis of 193 confirmed cases in Thailand.   PLoS Negl Trop Dis. 2020;14(10):e0008806-e0008806. doi:10.1371/journal.pntd.0008806PubMedGoogle Scholar
87.
Pung  R, Chiew  CJ, Young  BE,  et al; Singapore 2019 Novel Coronavirus Outbreak Research Team.  Investigation of three clusters of COVID-19 in Singapore: implications for surveillance and response measures.   Lancet. 2020;395(10229):1039-1046. doi:10.1016/S0140-6736(20)30528-6PubMedGoogle Scholar
88.
Qi  B, Peng  H, Shou  K,  et al.  Protecting healthcare professionals during the COVID-19 pandemic.   Biomed Res Int. 2020;2020:8469560-8469560. doi:10.1155/2020/8469560PubMedGoogle Scholar
89.
Qian  GQ, Yang  NB, Ding  F,  et al.  Epidemiologic and clinical characteristics of 91 hospitalized patients with COVID-19 in Zhejiang, China: a retrospective, multi-centre case series.   QJM. 2020;113(7):474-481. doi:10.1093/qjmed/hcaa089PubMedGoogle Scholar
90.
Qiu  C, Deng  Z, Xiao  Q,  et al.  Transmission and clinical characteristics of coronavirus disease 2019 in 104 outside-Wuhan patients, China.   J Med Virol. 2020;92(10):2027-2035. doi:10.1002/jmv.25975PubMedGoogle Scholar
91.
Ratovoson  R, Razafimahatratra  R, Randriamanantsoa  L,  et al.  Household transmission of COVID-19 among the earliest cases in Antananarivo, Madagascar.   Influenza Other Respir Viruses. 2022;16(1):48-55. doi:10.1111/irv.12896PubMedGoogle Scholar
92.
Ren  X, Li  Y, Yang  X,  et al.  Evidence for pre-symptomatic transmission of coronavirus disease 2019 (COVID-19) in China.   Influenza Other Respir Viruses. 2021;15(1):19-26. doi:10.1111/irv.12787PubMedGoogle Scholar
93.
Samrah  SM, Al-Mistarehi  A-H, Kewan  T,  et al.  Viral clearance course of COVID-19 outbreaks.   J Multidiscip Healthc. 2021;14:555-565. doi:10.2147/JMDH.S302891PubMedGoogle Scholar
94.
Sanche  S, Lin  YT, Xu  C, Romero-Severson  E, Hengartner  N, Ke  R.  High contagiousness and rapid spread of severe acute respiratory syndrome coronavirus 2.   Emerg Infect Dis. 2020;26(7):1470-1477. doi:10.3201/eid2607.200282PubMedGoogle Scholar
95.
Shen  Q, Guo  W, Guo  T,  et al.  Novel coronavirus infection in children outside of Wuhan, China.   Pediatr Pulmonol. 2020;55(6):1424-1429. doi:10.1002/ppul.24762PubMedGoogle Scholar
96.
Shen  Y, Xu  W, Li  C,  et al.  A cluster of novel coronavirus disease 2019 infections indicating person-to-person transmission among casual contacts from social gatherings: an outbreak case-contact investigation.   Open Forum Infect Dis. 2020;7(6):ofaa231. doi:10.1093/ofid/ofaa231Google Scholar
97.
Shi  J-C, Yu  Z-J, He  G-Q,  et al.  Epidemiological features of 105 patients infected with the COVID-19.   J Natl Med Assoc. 2021;113(2):212-217. doi:10.1016/j.jnma.2020.09.151PubMedGoogle Scholar
98.
Shi  P, Gao  Y, Shen  Y,  et al.  Characteristics and evaluation of the effectiveness of monitoring and control measures for the first 69 patients with COVID-19 from 18 January 2020 to 2 March in Wuxi, China.   Sustain Cities Soc. 2021;64:102559-102559. doi:10.1016/j.scs.2020.102559PubMedGoogle Scholar
99.
Shiel  E, Miyakis  S, Tennant  E,  et al.  Clinical characteristics and outcomes of COVID-19 in a low-prevalence, well resourced setting, Sydney, Australia.   Intern Med J. 2021;51(10):1605-1613. doi:10.1111/imj.15445PubMedGoogle Scholar
100.
Shu  Y, Deng  Z, Wang  H,  et al.  Epidemiological and clinical characteristics of 32 patients with COVID-19.   Can J Infect Control. 2020;19(08):679-686.Google Scholar
101.
Song  Q, Zhao  H, Fang  L, Liu  W, Zheng  C, Zhang  Y.  Study on assessing early epidemiological parameters of coronavirus disease epidemic in China.  Article in Chinese.  Chinese Journal of Epidemiology. 2020;41(4):461-465. doi:10.3760/cma.j.cn112338-20200205-00069Google Scholar
102.
Song  R, Han  B, Song  M,  et al.  Clinical and epidemiological features of COVID-19 family clusters in Beijing, China.   J Infect. 2020;81(2):e26-e30. doi:10.1016/j.jinf.2020.04.018PubMedGoogle Scholar
103.
Song  YS, Hao  YB, Liu  WW, Zhang  SS, Wang  P, Fan  TL.  Clinical features of 17 patients with 2019-nCoV.   Eur Rev Med Pharmacol Sci. 2020;24(20):10896-10901.PubMedGoogle Scholar
104.
Su  YJ, Kuo  KC, Wang  TW, Chang  CW.  Gender-based differences in COVID-19.   New Microbes New Infect. 2021;42:100905-100905. doi:10.1016/j.nmni.2021.100905PubMedGoogle Scholar
105.
Sugano  N, Ando  W, Fukushima  W.  Cluster of severe acute respiratory syndrome coronavirus 2 infections linked to music clubs in Osaka, Japan.   J Infect Dis. 2020;222(10):1635-1640. doi:10.1093/infdis/jiaa542PubMedGoogle Scholar
106.
Sun  B, Qi  Y, Chen  H,  et al.  Epidemiological profile of a cluster of COVID-19 caused by an imported case.  Article in Chinese. Chin J Prev Med. 2021;22(04):246-249.
107.
Sun  C, Zhang  XB, Dai  Y, Xu  XZ, Zhao  J.  [Clinical analysis of 150 cases of 2019 novel coronavirus infection in Nanyang City, Henan Province].  Article in Chinese.  Zhonghua Jie He He Hu Xi Za Zhi. 2020;43(6):503-508.PubMedGoogle Scholar
108.
Sun  L, Shen  L, Fan  J,  et al.  Clinical features of patients with coronavirus disease 2019 from a designated hospital in Beijing, China.   J Med Virol. 2020;92(10):2055-2066. doi:10.1002/jmv.25966PubMedGoogle Scholar
109.
Sun  YM, Liu  F, Cai  W,  et al.  [Transmission chains of clusters of COVID-19 associated with a market in Beijing].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2021;42(3):427-432.PubMedGoogle Scholar
110.
Sun  Y, Tian  L, Du  X, Wang  H, Li  Y, Wu  R.  Epidemiological and clinical characteristics of a familial cluster of COVID-19.   Epidemiol Infect. 2020;148:e145-e145. doi:10.1017/S0950268820001521PubMedGoogle Scholar
111.
Tan  WYT, Wong  LY, Leo  YS, Toh  MPHS.  Does incubation period of COVID-19 vary with age? a study of epidemiologically linked cases in Singapore.   Epidemiol Infect. 2020;148:e197-e197. doi:10.1017/S0950268820001995PubMedGoogle Scholar
112.
Tanaka  H, Ogata  T, Shibata  T,  et al.  Shorter incubation period among COVID-19 cases with the BA.1 Omicron variant.   Int J Environ Res Public Health. 2022;19(10):6330. doi:10.3390/ijerph19106330PubMedGoogle Scholar
113.
SARS-CoV-2 variant with lineage B.1.351 clusters investigation team.  Linked transmission chains of imported SARS-CoV-2 variant B.1.351 across mainland France, January 2021.   Euro Surveill. 2021;26(13):2100333. doi:10.2807/1560-7917.ES.2021.26.13.2100333Google Scholar
114.
Tian  S, Hu  N, Lou  J,  et al.  Characteristics of COVID-19 infection in Beijing.   J Infect. 2020;80(4):401-406. doi:10.1016/j.jinf.2020.02.018PubMedGoogle Scholar
115.
Tindale  LC, Stockdale  JE, Coombe  M,  et al.  Evidence for transmission of COVID-19 prior to symptom onset.   Elife. 2020;9:e57149. doi:10.7554/eLife.57149PubMedGoogle Scholar
116.
Viego  V, Geri  M, Castiglia  J, Jouglard  E.  Incubation period and serial interval of Covid-19 in a chain of infections in Bahia Blanca (Argentina).   Cien Saude Colet. 2020;25(9):3503-3510. doi:10.1590/1413-81232020259.20852020PubMedGoogle Scholar
117.
Wang  J, Wang  S, Yang  S,  et al.  Epidemiological characteristics of 17 coronavirus disease 2019 in Puyang, Henan.  Article in Chinese. Chinese Journal of Disease Control & Prevention. 2021;25(04):416-420.
118.
Wang  X, Gao  J, Wang  X, Hu  W, Liu  H.  Clinical and epidemiological characteristics of patients with COVID.   Can J Infect Control. 2020;19(03):223-226. doi:10.3389/fpubh.2020.00244Google Scholar
119.
Wang  X, Pan  Y, Zhang  D,  et al.  Basic epidemiological parameter values from data of real-world in mega-cities: the characteristics of COVID-19 in Beijing, China.   BMC Infect Dis. 2020;20(1):526-526. doi:10.1186/s12879-020-05251-9PubMedGoogle Scholar
120.
Wang  X, Zhou  Q, He  Y,  et al.  Nosocomial outbreak of COVID-19 pneumonia in Wuhan, China.   Eur Respir J. 2020;55(6):2000544. doi:10.1183/13993003.00544-2020PubMedGoogle Scholar
121.
Wang  Y, Liao  B, Guo  Y,  et al.  Clinical characteristics of patients infected with the novel 2019 coronavirus (SARS-Cov-2) in Guangzhou, China.   Open Forum Infect Dis. 2020;7(6):ofaa187. doi:10.1093/ofid/ofaa187PubMedGoogle Scholar
122.
Wei  X.  Analysis of clinical characteristics of 28 cases of COVID-19.  Article in Chinese.  Chinese Journal of Integrated Traditional and Western Medicine. 2021;28(3).Google Scholar
123.
Wei  Y, Zeng  W, Huang  X,  et al.  Clinical characteristics of 276 hospitalized patients with coronavirus disease 2019 in Zengdu District, Hubei Province: a single-center descriptive study.   BMC Infect Dis. 2020;20(1):549-549. doi:10.1186/s12879-020-05252-8PubMedGoogle Scholar
124.
Won  YS, Kim  J-H, Ahn  CY, Lee  H.  Subcritical transmission in the early stage of COVID-19 in Korea.   Int J Environ Res Public Health. 2021;18(3):1265. doi:10.3390/ijerph18031265PubMedGoogle Scholar
125.
Wong  J, Chaw  L, Koh  WC,  et al.  Epidemiological investigation of the first 135 COVID-19 cases in Brunei: implications for surveillance, control, and travel restrictions.   Am J Trop Med Hyg. 2020;103(4):1608-1613. doi:10.4269/ajtmh.20-0771PubMedGoogle Scholar
126.
Wu  J, Chen  X, Gong  L,  et al.  Epidemiological and clinical features of SARS-CoV-2 cluster infection in Anhui Province, Eastern China.   Int J Infect Dis. 2021;117:372-377. doi:10.1016/j.ijid.2021.04.064Google Scholar
127.
Wu  J, Xia  J, Yang  Q,  et al.  Clinical characteristics of SARS-CoV-2 infection at early stage.  Article in Chinese.  Chinese Journal of Infection and Chemotherapy. 2020;20(05):521-524.Google Scholar
128.
Wu  W, Liu  Y, Wei  Z,  et al.  Investigation and analysis on characteristics of a cluster of COVID-19 associated with exposure in a department store in Tianjin.  Article in Chinese.  Chin J Epidemiol. 2020;41(4):489-493. doi:10.3760/cma.j.cn112338-20200221-00139Google Scholar
129.
Wu  Y.  Epidemiological and clinical characteristics of 41 patients with COVID-19.  Article in Chinese.  Chinese Journal of Integrated Traditional and Western Medicine. 2020;27(4).Google Scholar
130.
Xiao  F, Chen  B, Xiao  T, Lee  SK, Yan  K, Hu  L.  Children with SARS-CoV-2 infection during an epidemic in China (outside of Hubei province).   Ann Transl Med. 2020;8(14):849-849. doi:10.21037/atm-20-2908PubMedGoogle Scholar
131.
Xiao  M, Hou  M, Liu  X, Li  Z, Zhao  Q.  Clinical characteristics of 71 patients with coronavirus disease 2019.  Article in Chinese.  Zhong Nan Da Xue Xue Bao Yi Xue Ban. 2020;45(7):790-796. doi:10.11817/j.issn.1672-7347.2020.200187PubMedGoogle Scholar
132.
Xiao  Z, Guo  W, Luo  Z, Liao  J, Wen  F, Lin  Y.  Examining geographical disparities in the incubation period of the COVID-19 infected cases in Shenzhen and Hefei, China.   Environ Health Prev Med. 2021;26(1):10-10. doi:10.1186/s12199-021-00935-3PubMedGoogle Scholar
133.
Xiao  Z, Xie  X, Guo  W,  et al.  Examining the incubation period distributions of COVID-19 on Chinese patients with different travel histories.   J Infect Dev Ctries. 2020;14(4):323-327. doi:10.3855/jidc.12718PubMedGoogle Scholar
134.
Xie  S, Zhang  G, Yu  H,  et al.  The epidemiologic and clinical features of suspected and confirmed cases of imported 2019 novel coronavirus pneumonia in north Shanghai, China.   Ann Transl Med. 2020;8(10):637-637. doi:10.21037/atm-20-2119PubMedGoogle Scholar
135.
Xin  H, Li  Y, Wu  P,  et al.  Estimating the latent period of coronavirus disease 2019 (COVID-19).   Clin Infect Dis. 2022;74(9):1678-1681. doi:10.1093/cid/ciab746Google Scholar
136.
Xu  T, Chen  C, Zhu  Z,  et al.  Clinical features and dynamics of viral load in imported and non-imported patients with COVID-19.   Int J Infect Dis. 2020;94:68-71. doi:10.1016/j.ijid.2020.03.022Google Scholar
137.
Xu  X-W, Wu  X-X, Jiang  X-G,  et al.  Clinical findings in a group of patients infected with the 2019 novel coronavirus (SARS-Cov-2) outside of Wuhan, China: retrospective case series.   BMJ. 2020;368:m606-m606. doi:10.1136/bmj.m606PubMedGoogle Scholar
138.
Yang  HY, Xu  J, Li  Y,  et al.  [The preliminary analysis on the characteristics of the cluster for the COVID-19].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(5):623-628.PubMedGoogle Scholar
139.
Yang  J, Wu  K, Ding  A,  et al.  Clinical characteristics, treatment, and prognosis of 74 2019 novel coronavirus disease patients in Hefei: a single-center retrospective study.   Medicine (Baltimore). 2021;100(21):e25645-e25645. doi:10.1097/MD.0000000000025645PubMedGoogle Scholar
140.
Yang  N, Shen  Y, Shi  C,  et al.  In-flight transmission cluster of COVID-19: a retrospective case series.   Infect Dis (Lond). 2020;52(12):891-901. doi:10.1080/23744235.2020.1800814PubMedGoogle Scholar
141.
You  C, Deng  Y, Hu  W,  et al.  Estimation of the time-varying reproduction number of COVID-19 outbreak in China.   Int J Hyg Environ Health. 2020;228:113555-113555. doi:10.1016/j.ijheh.2020.113555PubMedGoogle Scholar
142.
Yu  S, Cui  S, Rui  J,  et al.  Epidemiological characteristics and transmissibility for SARS-CoV-2 of population level and cluster level in a Chinese city.   Front Public Health. 2022;9:799536-799536. doi:10.3389/fpubh.2021.799536PubMedGoogle Scholar
143.
Yu  X, Sun  X, Cui  P,  et al.  Epidemiological and clinical characteristics of 333 confirmed cases with coronavirus disease 2019 in Shanghai, China.   Transbound Emerg Dis. 2020;67(4):1697-1707. doi:10.1111/tbed.13604PubMedGoogle Scholar
144.
Zhang  H, Song  S, Chen  Z,  et al.  A Cluster transmission of coronavirus disease 2019 and the prevention and control measures in the early stage of the epidemic in Xi’an, China, 2020.   Med Sci Monit. 2021;27:e929701-e929701. doi:10.12659/MSM.929701PubMedGoogle Scholar
145.
Zhang  J, Litvinova  M, Wang  W,  et al.  Evolving epidemiology and transmission dynamics of coronavirus disease 2019 outside Hubei province, China: a descriptive and modelling study.   Lancet Infect Dis. 2020;20(7):793-802. doi:10.1016/S1473-3099(20)30230-9PubMedGoogle Scholar
146.
Zhang  K, Zhang  Y, Li  M, Yu  M, Cha  L, Ruan  L.  Comparative analysis of clinical characteristics of imported and local family clusters of novel coronavirus pneumonia.  Article in Chinese.  Chinese Journal of Integrated Traditional and Western Medicine. 2020;27(4):385-389.Google Scholar
147.
Zhang  Q, Zhu  J, Jia  C, Xu  S, Jiang  T, Wang  S.  Epidemiology and clinical outcomes of COVID-19 patients in northwestern China who had a history of exposure in Wuhan City: departure time-originated pinpoint surveillance.   Front Med (Lausanne). 2021;8:582299-582299. doi:10.3389/fmed.2021.582299PubMedGoogle Scholar
148.
Zhang  T, Ding  S, Zeng  Z,  et al.  Estimation of incubation period and serial interval for SARS-CoV-2 in Jiangxi, China, and an updated meta-analysis.   J Infect Dev Ctries. 2021;15(3):326-332. doi:10.3855/jidc.14025PubMedGoogle Scholar
149.
Zhao  C, Xu  Y, Zhang  X,  et al.  Public health initiatives from hospitalized patients with COVID-19, China.   J Infect Public Health. 2020;13(9):1229-1236. doi:10.1016/j.jiph.2020.06.013PubMedGoogle Scholar
150.
Zhao  H, Zhang  Z, Lun  W,  et al.  Transmission dynamics and successful control measures of SARS-CoV-2 in the mega-size city of Guangzhou, China.   Medicine (Baltimore). 2021;100(48):e27846. doi:10.1097/MD.0000000000027846PubMedGoogle Scholar
151.
Zhao  S, Tang  B, Musa  SS,  et al.  Estimating the generation interval and inferring the latent period of COVID-19 from the contact tracing data.   Epidemics. 2021;36:100482-100482. doi:10.1016/j.epidem.2021.100482PubMedGoogle Scholar
152.
Zhao  WH, Ma  Y, Wang  H,  et al.  [Epidemiological characteristics of three local epidemics of COVID-19 in Guangzhou].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2021;42(12):2088-2095.PubMedGoogle Scholar
153.
Zhao  Z, Yin  M, Yin  S,  et al.  Analysis of clinical characteristics of 26 cases of COVID-19.  Article in Chinese.  Chinese Journal of Disease Control & Prevention. 2020;24(05):539-542.Google Scholar
154.
Zhong  S, Lin  F, Shi  L.  Clinical characteristics and outcomes of the patients with COVID-19: a report of 62 cases.  Article in Chinese.  Medical Journal of Chinese People’s Liberation Army. 2020;45(04):370-374.Google Scholar
155.
Zhu  J, Zhang  Q, Jia  C,  et al.  Challenges caused by imported cases abroad for the prevention and control of COVID-19 in China.   Front Med (Lausanne). 2021;8:573726-573726. doi:10.3389/fmed.2021.573726PubMedGoogle Scholar
156.
Zhu  N, Chen  S, Yang  G,  et al.  Analysis of the path from discovery of novel coronavirus disease 2019 cases to its diagnosis in Shaanxi Province.  Article in Chinese. Xi’an Jiaotong Daxue Xuebao Yixue Ban. 2020;41(06):923-926.
157.
Zhu  W, Zhang  M, Pan  J, Yao  Y, Wang  W.  Effects of prolonged incubation period and centralized quarantine on the COVID-19 outbreak in Shijiazhuang, China: a modeling study.   BMC Med. 2021;19(1):308-308. doi:10.1186/s12916-021-02178-zPubMedGoogle Scholar
158.
Wang  Y, Liao  B, Guo  Y,  et al.  Clinical characteristics of patients infected with the novel 2019 coronavirus (SARS-Cov-2) in Guangzhou, China.   Open Forum Infect Dis. 2020;7(6):ofaa187. doi:10.1093/ofid/ofaa187Google Scholar
159.
Elias  C, Sekri  A, Leblanc  P, Cucherat  M, Vanhems  P.  The incubation period of COVID-19: a meta-analysis.   Int J Infect Dis. 2021;104:708-710. doi:10.1016/j.ijid.2021.01.069Google Scholar
160.
Lessler  J, Reich  NG, Brookmeyer  R, Perl  TM, Nelson  KE, Cummings  DA.  Incubation periods of acute respiratory viral infections: a systematic review.   Lancet Infect Dis. 2009;9(5):291-300. doi:10.1016/S1473-3099(09)70069-6PubMedGoogle Scholar
161.
World Health Organization. Coronavirus disease (COVID-19). Updated May 13, 2021. Accessed April 17, 2022. https://www.who.int/emergencies/diseases/novel-coronavirus-2019/question-and-answers-hub/q-a-detail/coronavirus-disease-covid-19
162.
Cao  Y, Wang  J, Jian  F,  et al.  Omicron escapes the majority of existing SARS-CoV-2 neutralizing antibodies.   Nature. 2022;602(7898):657-663. doi:10.1038/s41586-021-04385-3PubMedGoogle Scholar
163.
US Centers for Disease Control and Prevention. Quarantine and Isolation. Updated March 30, 2022. Accessed April 17, 2022. https://www.cdc.gov/coronavirus/2019-ncov/your-health/quarantine-isolation.html
164.
Cowling  BJ, Muller  MP, Wong  IO,  et al.  Alternative methods of estimating an incubation distribution: examples from severe acute respiratory syndrome.   Epidemiology. 2007;18(2):253-259. doi:10.1097/01.ede.0000254660.07942.fbPubMedGoogle Scholar
165.
Chen  Y, Klein  SL, Garibaldi  BT,  et al.  Aging in COVID-19: vulnerability, immunity and intervention.   Ageing Res Rev. 2021;65:101205-101205. doi:10.1016/j.arr.2020.101205PubMedGoogle Scholar
166.
Virlogeux  V, Fang  VJ, Wu  JT,  et al.  Brief report: incubation period duration and severity of clinical disease following severe acute respiratory syndrome coronavirus infection.   Epidemiology. 2015;26(5):666-669. doi:10.1097/EDE.0000000000000339PubMedGoogle Scholar
167.
Virlogeux  V, Park  M, Wu  JT, Cowling  BJ.  Association between severity of MERS-CoV infection and incubation period.   Emerg Infect Dis. 2016;22(3):526-528. doi:10.3201/eid2203.151437PubMedGoogle Scholar
2 Comments for this article
EXPAND ALL
Beyond the mean COVID-19 incubation period
Boris Bikbov, MD, PhD | Istituto di Ricerche Farmacologiche Mario Negri IRCCS
The systematic review presented by Wu et al.[1] demonstrates a decrease in the mean COVID-19 incubation period that supports change in clinical features of viral transmission of different SARS-CoV-2 strains and faster spread. These findings are important, however, the analysis could be more representative if considers several aspects.

First, the observed between-studies heterogeneity, with I2 = 98.8% for the pooled analysis and similar I2 for separate SARS-CoV-2 strains, indicates substantial differences in the primary studies population, and suggests to include in the analysis moderator variables[2] or to implement a meta-analysis with individual patient-level data. The manuscript does not indicate
whether it was not performed due to difficulties with original data provision or due to other reasons. While the individual-level meta-analysis is more time consuming and requires communication with teams that conducted primary studies, it could explain the observed between-studies heterogeneity and provide insights about the incubation period in certain subgroups. Moreover, the inclusion of individual-level data allows to estimate the group variability of the incubation period, instead of just reporting its mean values and their uncertainty. Particularly, it could estimate the proportion of patients with the incubation period exceeding 14 days, that was detected in 5-10% of patients by earlier studies[3],[4],[5], and has not yet been analyzed for the recently emerged strains. Additionally, reporting of not only the mean but also the median incubation period should be obligatory, because these indicators could differ substantially.[2]

Second, it would be worth to discuss the reasons for the difference in the mean incubation period between 6.6 days observed in the pooled analysis, and substantially lower values of 5.0, 4.5, 4.4, and 3.4 days in studies with known SARS-CoV-2 strains representing Alpha, Beta, Delta, and Omicron variants, respectively. The difference in 1.6-3.2 days between studies that accounted or not accounted for the information about strains could reflect not only the biological features of viral variants but could also be explained by some selection bias of patients with evaluated strains. The analysis not sufficiently considers factors related to mean incubation period for SARS-CoV-2 strains. Thus, another meta-analysis suggested that ethnicity could play a role, and the mean COVID-19 incubation period was 6.5 days (95% CI 6.1–6.9) and 4.6 days (95% CI 4.1–5.1) in patients residing in or out of the mainland China, respectively.[5] Age could lead not only to a linear increase,[6] but a more complex U-shaped correlation with incubation period with higher values in both children and elderly persons compared with middle-age adults. [5]

Third, in the discussion, the authors attributed only to children two features that were demonstrated in adults: the pre-symptomatic SARS-CoV-2 shedding that leads to the possibility of virus transmission even during the late incubation period,[7] and incubation period exceeding 14 days.[3],[4],[5]

[1] Wu et al. JAMA Netw Open 2022; 10.1001/jamanetworkopen.2022.28008.
[2] Dhouib et al. Syst Rev 2021; 10.1186/s13643-021-01648-y.
[3] Qin et al. Sci Adv 2020; 10.1126/sciadv.abc1202.
[4] Bikbov B, Bikbov A. Travel Med Infect Dis 2021; 10.1016/j.tmaid.2021.101976.
[5] Cheng et al. Infect Dis Poverty 2021; 10.1186/s40249-021-00901-9.
[6] Wei et al. Infection 2021; 10.1007/s15010-021-01682-x.
[7] Casey-Bryars et al. BMJ Open 2021; 10.1136/bmjopen-2020-041240.

CONFLICT OF INTEREST: None Reported
READ MORE
Reriability of data
Tatsuyoshi Ikenoue, MD, MPH, PhD | Shiga University
I believe this systematic review[1] is very important for considering trends in infection. It is noteworthy that, as Dr. Bikbov points out, the incubation period is getting shorter with each generation. However, the values listed raise limitations. The confidence intervals for the alpha variant are very narrow. The value of s is 2.4, which is extremely small when calculated from the given n and the confidence interval width. (Beta: 4.3, Delta 16.0, Omicron: 7.9) There was only one study about the alpha variant, but it does not seem to explain this event entirely. 

[1] Wu et al. JAMA Netw
Open 2022; 10.1001/jamanetworkopen.2022.28008.
CONFLICT OF INTEREST: None Reported
READ MORE
Original Investigation
Infectious Diseases
August 22, 2022

Incubation Period of COVID-19 Caused by Unique SARS-CoV-2 Strains: A Systematic Review and Meta-analysis

Author Affiliations
  • 1Department of Epidemiology and Biostatics, School of Public Health, Peking University, Beijing, China
  • 2Vanke School of Public Health, Tsinghua University, Beijing, China
JAMA Netw Open. 2022;5(8):e2228008. doi:10.1001/jamanetworkopen.2022.28008
Key Points

Question  What are the incubation periods of COVID-19 caused by different SARS-CoV-2 strains?

Findings  In this systematic review and meta-analysis of 141 articles, the pooled incubation period was 6.57 days. The incubation periods of COVID-19 caused by the Alpha, Beta, Delta, and Omicron variants were 5.00, 4.50, 4.41, and 3.42 days, respectively.

Meaning  These results suggest that with the evolution of mutant strains, the incubation period of COVID-19 decreased gradually from Alpha variant to Omicron variant.

Abstract

Importance  Several studies were conducted to estimate the average incubation period of COVID-19; however, the incubation period of COVID-19 caused by different SARS-CoV-2 variants is not well described.

Objective  To systematically assess the incubation period of COVID-19 and the incubation periods of COVID-19 caused by different SARS-CoV-2 variants in published studies.

Data Sources  PubMed, EMBASE, and ScienceDirect were searched between December 1, 2019, and February 10, 2022.

Study Selection  Original studies of the incubation period of COVID-19, defined as the time from infection to the onset of signs and symptoms.

Data Extraction and Synthesis  Following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guideline, 3 reviewers independently extracted the data from the eligible studies in March 2022. The parameters, or sufficient information to facilitate calculation of those values, were derived from random-effects meta-analysis.

Main Outcomes and Measures  The mean estimate of the incubation period and different SARS-CoV-2 strains.

Results  A total of 142 studies with 8112 patients were included. The pooled incubation period was 6.57 days (95% CI, 6.26-6.88) and ranged from 1.80 to 18.87 days. The incubation period of COVID-19 caused by the Alpha, Beta, Delta, and Omicron variants were reported in 1 study (with 6374 patients), 1 study (10 patients), 6 studies (2368 patients) and 5 studies (829 patients), respectively. The mean incubation period of COVID-19 was 5.00 days (95% CI, 4.94-5.06 days) for cases caused by the Alpha variant, 4.50 days (95% CI, 1.83-7.17 days) for the Beta variant, 4.41 days (95% CI, 3.76-5.05 days) for the Delta variant, and 3.42 days (95% CI, 2.88-3.96 days) for the Omicron variant. The mean incubation was 7.43 days (95% CI, 5.75-9.11 days) among older patients (ie, aged over 60 years old), 8.82 days (95% CI, 8.19-9.45 days) among infected children (ages 18 years or younger), 6.99 days (95% CI, 6.07-7.92 days) among patients with nonsevere illness, and 6.69 days (95% CI, 4.53-8.85 days) among patients with severe illness.

Conclusions and Relevance  The findings of this study suggest that SARS-CoV-2 has evolved and mutated continuously throughout the COVID-19 pandemic, producing variants with different enhanced transmission and virulence. Identifying the incubation period of different variants is a key factor in determining the isolation period.

Introduction

In December 2019, multiple cases of novel coronavirus disease (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), were reported in Wuhan, China.1 On March 11, 2020, the World Health Organization (WHO) declared that COVID-19 can be characterized as a pandemic. To date, the transmission of COVID-19 is still difficult to contain, as confirmed and death cases are still increasing. Up to March 16, 2022, 460 280 168 confirmed cases and 6 050 018 confirmed deaths have been reported to the WHO.2 Rapid spread of COVID-19 has had enormous social, economic, and health care system effects around the world. Effective treatment to block the spread of COVID-19 is not developed yet, so countries have implemented a series of nontreatment interventions such as social distancing, isolation, face mask mandates, and quarantining to reduce its rapid transmission.3 Existing evidence has shown that most of COVID-19 cases are missed by screening because infected persons are unaware they were exposed and have not developed symptoms yet.4-6

Incubation period is one of the most important epidemiological parameters of infectious diseases. Knowledge of the disease’s incubation period is of great significance for case definition, management of emerging threats, estimation of the duration of follow-up for contact tracing and secondary case detection, and the establishment of public health programs aimed at reducing local transmission.7 Previous studies8 have reported that the average serial interval of COVID-19 is shorter than the average incubation period, which suggests a substantial proportion of presymptomatic transmission. For diseases caused by different pathogens, the length of incubation period is the key factor to determine the isolation period of infected persons.

Since the beginning of the COVID-19 epidemic, SARS-CoV-2 has evolved and mutated continuously, producing variants with different transmissibility and virulence. SARS-CoV-2 variants are classified by the WHO into 2 types: variants of concern (VOC) and variants of interest (VOI).9 According to the US Center for Disease Control and Prevention (CDC), a VOC is a variant that has increased transmissibility, increased virulence, a resistance to vaccine or acquired immunity from previous infection, and has the ability to elude diagnostic detection.10 Several VOC have emerged from the original wild-type strain isolated in Wuhan since the outbreak first began in December 2019, such as Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), Delta (B.1.617.2), and Omicron (B.1.1.529).9 The Alpha variant was first detected in the UK in September 2020; the Beta variant in South Africa in May 2020; and the Gamma variant in Brazil in September 2020. All 3 quickly became the main virus strains worldwide.

Globally, many studies were conducted to estimate the average incubation period of COVID-19. However, the reported estimates of incubation period in these fragmented studies vary depending on the number of study participants recruited, the type of design employed, the data collection period, and the country in which the study was conducted. In addition, with the spread of the Delta and Omicron variants, the current incubation period of COVID-19 is different from that in the outbreak of Wuhan. This meta-analysis was aimed to determine the overall pooled incubation period of COVID-19 and the incubation period of COVID-19 caused by different SARS-CoV-2 variants using available evidence, so as to adjust prevention and control strategies and better block the transmission of COVID-19.

Methods
Search Strategy

We conducted this meta-analysis following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. This review was not registered. This study was exempted from ethics review board at Peking University because it used previously published literature in its analysis. A survey of the literature was implemented between December 1, 2019, and February 10, 2022. Publications on the electronic databases PubMed, Embase and ScienceDirect were searched using the keywords novel coronavirus, SARS-CoV-2, 2019-nCoV, or COVID-19 and either incubation period or incubation (eTable 1 in the Supplement). No restrictions on language or publication status were imposed so long as an English abstract was available. The initial searches were carried out by 3 of the investigators (Y.W., L.K., R.G.).

Inclusion and Exclusion Criteria

Inclusion criteria for selecting the studies were that the incubation period was one of the primary outcomes of the study and that, when the incubation periods of multiple groups were reported in the same study, only the group with the largest study population was included. Criteria for exclusion included articles not conducted as studies (ie, editorials, perspective articles, letters to the editor, reviews, article information, or comments), duplicate studies, and articles with overlapping study populations (ie, enrolling the same population in the same region around the same period).

Outcome Measures and Study Selection

The outcome variable was the mean estimate of the incubation period. Incubation period was defined as the time from when the infection occurred to the onset of signs and symptoms or the first positive test. It was measured with cases of a well-defined period of exposure and symptom onset.

Results of searches were screened in 2 stages. First, titles and abstracts were screened and only relevant articles retained. Next, articles were read in detail—studies were selected for meta-analysis if they reported either results fitting our primary parameters (with CIs) or sufficient information to facilitate calculation of those values.

After screening for inclusion and exclusion criteria, data extraction was carried out from the included studies. The name of the first author, area of study, time period for data collection, characteristics of the study population, strain type, and estimates for the incubation period with 95% CI were extracted from the selected studies. Ninety-five percent CIs were estimated for the studies reporting mean with standard deviation by using the following formula, which is generally used to calculate the 95% CI for any parameter:

Image description not available.

where μ indicates the mean incubation period, s the standard deviation, and n the sample size of the study. Some studies reported only median with interquartile range or range. Mean and the standard deviation were calculated for such studies by using an appropriate approximation for the consistency in synthesizing the results for meta-analysis.11,12

Quality Assessment

Once studies were shortlisted, 2 authors (Y.W., L.K.) independently conducted appraisals of study quality. We used a scale modified from the Newcastle-Ottawa scale13 by McAloon et al14 to assess the quality of observational studies in meta-analyses (eTable 2 in the Supplement). This scale consists of 2 parts with a full score of 5 stars. The first part is external validity, with a maximum of 1 star; the second part is internal validity, which includes exposure window (a maximum of 2 stars) and outcomes with (a maximum of 2 stars). Based on the combined score of these 2 parts, each paper was categorized as either weak (1 star or less), moderate (2 to 3 stars), or strong (4 stars or more). After the studies were evaluated by the 2 authors, the results were compared and differences in ratings were resolved by discussion until a consensus rating was agreed upon.

Statistical Analysis

A meta-analysis of continuous outcomes was employed for this study. We analyzed the data sets for the incubation period. After extracting all essential data using Excel 2021 (Microsoft Corporation), data were exported to Stata version 14.1 (StataCorp) statistical software for meta-analysis. A random-effect meta-analysis with an estimation of DerSimonian and Laird method was performed. Pooled mean estimates with 95% CIs were presented using forest plots. To determine the extent of variation between the studies, we conducted a heterogeneity test using the Higgins method, that was quantified by I2 value.15 Publication bias was also assessed using a funnel plot. A 2-sided P < .05 was considered statistically significant.

Results
Search Results

We identified 5012 records through PubMed, EMBASE, and Science Direct database searches, and documented the study selection process in a flowchart and showed the total numbers of retrieved references and the numbers of included and excluded studies (Figure 1). Based on the inclusion and exclusion criteria, 142 articles (8112 patients) were selected for analysis.16-157

Study Characteristics

Over the 142 studies, the quality assessment gave 45 strong, 82 moderate, and 15 weak studies (eTable 3 in the Supplement). Most of the studies (93 [65.5%]) were conducted between January and March 2020 and most were conducted in China (108 [76.1%]). One study used case data from multiple countries around the world,81 6 studies were conducted in South Korea,37,46,51,52,59,124 4 in France,25,29,35,113 3 in Japan,80,105 2 in Singapore,87,111 2 in India,16,78,83,103 2 in Vietnam,21,58 and 2 in Australia.45,99 One hundred nineteen studies (83.8%) included patients infected with the wild-type strain, 5 (3.5%) with the multiple strains,17,26,35,80,112 and 11 (7.7%) with an unknown strain84,93,99,106,116,126,135,142,148,150,157 (eTable 4 in the Supplement).

Pooled Average Estimate of Incubation Period

The mean incubation period of COVID-19 was 6.57 days (95% CI, 6.26-6.88 days), ranging from 1.80 to 18.87 days (Table). There was substantial heterogeneity between the studies (I2 = 98.8%; P < .001). Our results suggested no potential publication bias in the included studies (eFigure 1 in the Supplement). The standard error for all the included studies in the meta-analysis was very low except for a 2020 study conducted by Xie134 where the highest standard error was observed.

Mean Incubation Periods of COVID-19 Infected by Different Strains

Across a total of 119 studies with data on the wild-type strain, the mean incubation period was 6.65 days (95% CI, 6.31-6.99) (eFigure 2 in the Supplement). For infections caused by the Alpha variant, an incubation period of 5.00 days (95% CI, 4.94-5.06) was reported in a single study.35 One study from France reported the incubation period of 4.50 days (95% CI, 1.83-7.17 days) for COVID-19 caused by the Beta variant.113 Another study reported the incubation period of COVID-19 caused by the Beta/Gamma variant was 5.10 days (95% CI, 4.87-5.33 days).35

A total of 6 studies reported the incubation period of COVID-19 caused by the Delta variant, including 2 from China,63,152 2 from Japan,80,112 1 from France,35 and 1 from Spain,26 with a pooled incubation period of 4.41 days (95% CI, 3.76-5.05 days) (Figure 2). Five studies reported the incubation period of COVID-19 caused by the Omicron variant—1 each from Norway,20 Spain,26 Japan,112 the Netherlands,17 and South Korea59—with a pooled incubation period of 3.42 days (95% CI, 2.88-3.96 days) (Figure 2). With the evolution of the mutant strains, the incubation period of COVID-19 appeared to decrease gradually from the Alpha variant to Omicron variant, but there was no significant difference between the groups.

Subgroup Analysis

A total of 8 studies reported the incubation period of COVID-19 among older patients (ie, aged 60 years or more).24,37,38,44,54,68,105,111 The pooled mean incubation period for these studies was 7.43 days (95% CI, 5.75-9.11 days), which was slightly higher than the pooled incubation period of the general population (6.65 days; 95% CI, 6.34-6.96 days), but the difference was not significant (Figure 3).

The mean incubation period of COVID-19 among infected children (under ages 18 years) was 8.82 days (95% CI, 8.19-9.45 days) across 8 studies,36,39,41,44,75,95,102,130 which was higher than the pooled incubation period of the general population (6.65 days; 95% CI, 6.34-6.96 days), and the difference was significant (P < .001) (Figure 3).

Five studies reported the incubation period in patients with nonsevere illness,44,70,121,123,139 with a pooled value of 6.99 days (95% CI, 6.07-7.92 days). Five studies analyzed the incubation period of patients with severe disease,27,70,121,123,139 with a pooled value of 6.69 days (95% CI, 4.53-8.85 days), which was slightly shorter than that of patients with nonsevere illness, but the difference was not significant (Figure 4).

Discussion

Our findings suggested that COVID-19 had a mean incubation period of 6.57 days (95% CI, 6.26-6.88 days), which was similar to the results of Elias et al159 (6.38 days; 95% CI, 5.79-6.97 days) and McAloon et al14 (6.5 days, 95% CI, 5.9-7.1 days). COVID-19 seemed to have a longer incubation period than that of other acute respiratory viral infections such as human coronavirus (3.2 days), influenza A (1.43-1.64 days), parainfluenza (2.6 days), respiratory syncytial virus (4.4 days), and rhinovirus (1.4 days).160 Furthermore, the median incubation period for SARS in 2009 had been estimated as 4.0 days,160 which was lower than COVID-19. In this study, the shortest mean incubation reported was 1.8 days and the longest incubation was 18.87 days. At present, based on the assumption that the incubation period of COVID-19 is 1 to 14 days, the WHO still recommends that COVID-19 close contacts be isolated for 14 days.161

Our study found that the incubation period of COVID-19 caused by Alpha variant was 5.00 days (95% CI, 4.94-5.06 days), and the incubation period of COVID-19 caused by Beta variant was 4.50 days (95% CI, 1.83-7.17 days), which were similar to that of the wild-type strain in Wuhan, China (5.2 days).64,145

The Delta variant, which was first reported in India in October 2020, was dominant in the second wave of COVID-19 outbreak in India in May 2021.9 Our study revealed that the incubation period of COVID-19 caused by Delta variant was 4.41 days (95% CI, 3.76-5.05), which was shorter than the pooled incubation period of COVID-19 (6.26 days), and also shorter than that caused by Alpha variant and Beta variants.

On November 24, 2021, South Africa first discovered and reported a case of Omicron variant infection to the WHO. Since then, this variant has quickly become the main virus strain in South Africa and spread to many countries and regions around the world. The Omicron variant is exceptional for carrying over 30 mutations in the spike glycoprotein, which have been predicted to influence antibody neutralization and spike function.162 Our study revealed that the incubation period of COVID-19 caused by Omicron variant was 3.42 days (95% CI, 2.88-3.96 days), which was shorter than the Alpha, Beta, and Delta variants. The CDC released new quarantine and isolation policy on March 30, 2022, which stated that people exposed to COVID-19 should stay home and away from other people for at least 5 days.163 At present, some countries around the world require close contacts to be isolated for 14 days. However, with the shortening of the incubation period of new variants, the isolation period can be adjusted appropriately to reduce the pressure on the health system.

Eight studies reported the incubation period among older patients (ages 60 years and older), and the mean incubation period of older patients was about 7.43 days (95% CI, 5.75-9.11), which was slightly higher than the pooled incubation period among the general population. Although the difference between the incubation periods of older patients and the overall incubation period was not significant, there was still a lot of evidence to support the hypothesis of a longer incubation period in older populations due to a slower immune response among older patients. Cowling et al164 hypothesized about this in their report on SARS in 2007, where they demonstrated that older patients had longer incubation periods, suggesting that this might have resulted from a delayed immune response. A study by Chen et al165 revealed that several SARS-CoV nonstructural proteins that were shared by SARS-CoV-2 suppress the type 1 interferon response, and such suppression was shown to lead to poor CD8+ T-cell response to viral infection. Therefore, age-associated weaker type 1 interferon responses coupled with direct viral suppression could serve as a critical innate immune mechanism that leads to poor cell-mediated immunity and increased vulnerability of older adults to SARS-CoV-2 infection with therapeutic implication. Additionally, older patients were more likely to experience symptom minimization and be more likely to ignore early symptoms and only report later when symptoms become more severe or intolerable.111 The lack of a fever response in older patients, the nonspecific geriatric presentations in an infectious illness (such as falls and delirium), and multi-comorbidities might result in a delayed awareness of disease onset and its detection by a clinician.54

Additionally, our study also revealed that the mean incubation period for infected children (8.82 days; 95% CI, 8.19-9.45) was also longer than the pooled incubation period among the general population (6.65 days). Infected children tend to present with mild clinical symptoms without the classic phenotype of lung pneumonia, and COVID-19 symptoms are easily confused with other influenza-like illnesses, which renders infected children difficult to identify.130 Second, previous studies found that children can be a source of transmission during the viral incubation period. Some infected children may have an incubation period of more than 14 days. Indeed, it is difficult for investigators to collect information about the symptoms of very young children because they cannot accurately express their symptoms.130

Previous studies on SARS indicated that the incubation period of patients was related to the severity of the disease, and the incubation period of fatal cases was shorter.166 Virlogeux et al167 also found that Middle East Respiratory Syndrome patients with a shorter incubation period proceeded to have more severe disease. However, there are few studies on the association between the length of COVID-19 incubation period and the severity of infection. Our study found that the incubation period of COVID-19 in patients with severe illness was shorter (6.69 days) than patients with nonsevere illness. Studies have indicated that shorter incubation periods are associated with more serious disease, and this is related to the number of cells initially infected by the virus.123

This study was the first meta-analysis of the incubation period of COVID-19 caused by SARS-CoV-2 variants. We compared the incubation period of COVID-19 caused by different variants and the wild-type strain, and the results may be helpful in changing public health guidance on duration of quarantine, outbreak investigation, and contact tracing.

Limitations

This study had several limitations. First, by definition, the required case data for the determination of individual incubation periods need to include both exposure (window) and onset of symptoms. In most studies, the data were collected retrospectively, resulting in a recall bias (uncertain exact dates of exposure) that would inevitably influence our assessment. Second, the estimate of the incubation period was computed with data with considerable heterogeneity. Possible sources of heterogeneity included difference in study population, data collection period, and method of analysis. Wild-type strain studies were mostly from Chinese patients; while variants studies were not. Population factors, especially those related to public policy and social behavior, may be confounding variables. In this study, we assumed that the incubation period was consistent across populations. Third, there were few studies on the incubation period of COVID-19 caused by SARS-CoV-2 variants. Because of the urgent timeline for data extraction and analysis, these studies have estimated the incubation period in a limited case number in a short period of time, which necessitates the cautious interpretation of the generalizability of our findings. The numbers were too small to detect systematic differences in incubation time in regards to age or sex.

Conclusions

Although variants such as Alpha, Beta, and Gamma are currently only prevalent in a few countries in Southeast Asia, South America, and Africa, the Delta and Omicron variants have become the dominant strains in many countries around the world. Identifying the incubation period of different variants is a key factor in determining the isolation period. The pooled incubation period of COVID-19 in this study was 6.57 days. The incubation period for COVID-19 caused by the Alpha and Beta variants was approximately 5 days. The incubation period of COVID-19 caused by the Delta and Omicron variants was significantly shorter than that of the other variants.

Back to top
Article Information

Accepted for Publication: July 6, 2022.

Published: August 22, 2022. doi:10.1001/jamanetworkopen.2022.28008

Correction: This article was corrected on September 16, 2022, to fix a word describing study results in the Discussion.

Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2022 Wu Y et al. JAMA Network Open.

Corresponding Author: Min Liu, PhD, Department of Epidemiology and Biostatistics, School of Public Health, Peking University, No. 38, Xueyuan Road, Haidian District, Beijing 100191, China (liumin@bjmu.edu.cn); Wannian Liang, PhD, Vanke School of Public Health, Tsinghua University, Beijing 100084, China (liangwn@tsinghua.edu.cn).

Author Contributions: Drs Wu and Liu had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Wu, J. Liu, M. Liu.

Acquisition, analysis, or interpretation of data: Wu, Kang, Guo, M. Liu, Liang.

Drafting of the manuscript: Wu, Kang, Guo.

Critical revision of the manuscript for important intellectual content: Wu, J. Liu, M. Liu, Liang.

Statistical analysis: Wu, Guo.

Obtained funding: J. Liu, M. Liu, Liang.

Administrative, technical, or material support: M. Liu.

Supervision: Wu, Guo, J. Liu, M. Liu, Liang.

Conflict of Interest Disclosures: None reported.

Funding/Support: National Key Research and Development Program of China (2021ZD0114104, 2021ZD0114105, 2021ZD0114101); National Natural Science Foundation of China (71934002, 72122001).

Role of the Funder/Sponsor: The founders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Reference
1.
Guan  W-J, Ni  Z-Y, Hu  Y,  et al; China Medical Treatment Expert Group for Covid-19.  Clinical characteristics of coronavirus disease 2019 in China.   N Engl J Med. 2020;382(18):1708-1720. doi:10.1056/NEJMoa2002032PubMedGoogle ScholarCrossref
2.
World Health Organization. Coronavirus (COVID-19) Dashboard. 2022; https://covid19.who.int/.
3.
Sanders  JM, Monogue  ML, Jodlowski  TZ, Cutrell  JB.  Pharmacologic treatments for coronavirus disease 2019 (COVID-19): a review.   JAMA. 2020;323(18):1824-1836. doi:10.1001/jama.2020.6019PubMedGoogle ScholarCrossref
4.
Gostic  K, Gomez  AC, Mummah  RO, Kucharski  AJ, Lloyd-Smith  JO.  Estimated effectiveness of symptom and risk screening to prevent the spread of COVID-19.   Elife. 2020;9:9. doi:10.7554/eLife.55570PubMedGoogle ScholarCrossref
5.
Tawe Ngi  AM, Johnston  S, Albayat  SS,  et al.  Pre-symptomatic and asymptomatic transmission of COVID-19: Implications for control measures in Qatar.   Qatar Med J. 2021;2021(3):59-59.PubMedGoogle Scholar
6.
Ganyani  T, Kremer  C, Chen  D,  et al.  Estimating the generation interval for coronavirus disease (COVID-19) based on symptom onset data, March 2020.   Euro Surveill. 2020;25(17):2000257. doi:10.2807/1560-7917.ES.2020.25.17.2000257Google ScholarCrossref
7.
Nishiura  H, Mizumoto  K, Ejima  K, Zhong  Y, Cowling  B, Omori  R.  Incubation period as part of the case definition of severe respiratory illness caused by a novel coronavirus.   Euro Surveill. 2012;17(42):20296. PubMedGoogle ScholarCrossref
8.
Nishiura  H, Linton  NM, Akhmetzhanov  AR.  Serial interval of novel coronavirus (COVID-19) infections.   Int J Infect Dis. 2020;93:284-286. doi:10.1016/j.ijid.2020.02.060Google ScholarCrossref
9.
World Health Organization. Tracking SARS-CoV-2 variants. Updated July 19, 2022. Accessed April 17, 2022. https://www.who.int/en/activities/tracking-SARS-CoV-2-variants/
10.
US Centers for Disease Control and Prevention. SARS-CoV-2 Variant Classifications and Definitions. Updated April 26, 2021. Accessed April 17, 2022. https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-classifications.html#anchor_1632158885160
11.
Wan  X, Wang  W, Liu  J, Tong  T.  Estimating the sample mean and standard deviation from the sample size, median, range and/or interquartile range.   BMC Med Res Methodol. 2014;14:135. doi:10.1186/1471-2288-14-135PubMedGoogle ScholarCrossref
12.
Hozo  SP, Djulbegovic  B, Hozo  I.  Estimating the mean and variance from the median, range, and the size of a sample.   BMC Med Res Methodol. 2005;5:13. doi:10.1186/1471-2288-5-13PubMedGoogle ScholarCrossref
13.
Wells  G, Shea  B, O’Connell  D,  et al. The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analyses. Published 2021. Accessed April 17, 2022. http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp
14.
McAloon  C, Collins  Á, Hunt  K,  et al.  Incubation period of COVID-19: a rapid systematic review and meta-analysis of observational research.   BMJ Open. 2020;10(8):e039652-e039652. doi:10.1136/bmjopen-2020-039652PubMedGoogle ScholarCrossref
15.
Melsen  WG, Bootsma  MC, Rovers  MM, Bonten  MJ.  The effects of clinical and statistical heterogeneity on the predictive values of results from meta-analyses.   Clin Microbiol Infect. 2014;20(2):123-129. doi:10.1111/1469-0691.12494Google ScholarCrossref
16.
Areekal  B, Vijayan  SM, Suseela  MS.  Risk factors, epidemiological and clinical outcome of close contacts of COVID-19 cases in a tertiary hospital in southern India.  J Clin Diagn Res. 2021;15(3):LC34-LC37.
17.
Backer  JA, Eggink  D, Andeweg  SP,  et al.  Shorter serial intervals in SARS-CoV-2 cases with Omicron BA.1 variant compared with Delta variant, the Netherlands, 13 to 26 December 2021.   Euro Surveill. 2022;27(6):2200042. doi:10.2807/1560-7917.ES.2022.27.6.2200042Google ScholarCrossref
18.
Backer  JA, Klinkenberg  D, Wallinga  J.  Incubation period of 2019 novel coronavirus (2019-nCoV) infections among travelers from Wuhan, China, 20-28 January 2020.   Euro Surveill. 2020;25(5):2000062. doi:10.2807/1560-7917.ES.2020.25.5.2000062Google ScholarCrossref
19.
Bao  C, Pan  E, Ai  J,  et al.  COVID-19 outbreak following a single patient exposure at an entertainment site: an epidemiological study.   Transbound Emerg Dis. 2021;68(2):773-781. doi:10.1111/tbed.13742PubMedGoogle ScholarCrossref
20.
Brandal  LT, MacDonald  E, Veneti  L,  et al.  Outbreak caused by the SARS-CoV-2 Omicron variant in Norway, November to December 2021.   Euro Surveill. 2021;26(50):2101147. doi:10.2807/1560-7917.ES.2021.26.50.2101147Google ScholarCrossref
21.
Bui  LV, Nguyen  HT, Levine  H,  et al.  Estimation of the incubation period of COVID-19 in Vietnam.   PLoS One. 2020;15(12):e0243889-e0243889. doi:10.1371/journal.pone.0243889PubMedGoogle ScholarCrossref
22.
Chen  G, Wu  MZ, Qin  CJ,  et al.  Epidemiological analysis of 18 patients with COVID-19.   Eur Rev Med Pharmacol Sci. 2020;24(23):12522-12526.PubMedGoogle Scholar
23.
Zhang  W; Covid-Epidemiology Investigation Team; Laboratory Testing Team.  Local outbreak of COVID-19 in Shunyi District attributed to an asymptomatic carrier with a history of stay in Indonesia—Beijing municipality, China, December 23, 2020.   China CDC Wkly. 2021;3(10):214-217. doi:10.46234/ccdcw2020.062PubMedGoogle ScholarCrossref
24.
Dai  J, Yang  L, Zhao  J.  Probable longer incubation period for elderly COVID-19 cases: analysis of 180 contact tracing data in Hubei Province, China.   Risk Manag Healthc Policy. 2020;13:1111-1117. doi:10.2147/RMHP.S257907PubMedGoogle ScholarCrossref
25.
de Laval  F, Grosset-Janin  A, Delon  F,  et al.  Lessons learned from the investigation of a COVID-19 cluster in Creil, France: effectiveness of targeting symptomatic cases and conducting contact tracing around them.   BMC Infect Dis. 2021;21(1):457-457. doi:10.1186/s12879-021-06166-9PubMedGoogle ScholarCrossref
26.
Del Águila-Mejía  J, Wallmann  R, Calvo-Montes  J, Rodríguez-Lozano  J, Valle-Madrazo  T, Aginagalde-Llorente  A.  Secondary attack rate, transmission and incubation periods, and serial interval of SARS-CoV-2 Omicron variant, Spain.   Emerg Infect Dis. 2022;28(6):1224-1228. doi:10.3201/eid2806.220158PubMedGoogle ScholarCrossref
27.
Deng  L, Li  Z, Liu  Y,  et al.  Epidemiological and clinical findings of discharge patients infected with the 2019 novel coronavirus (SARS-CoV-2) in Changchun, Northeast China: a retrospective cohort study.  Acta Med Mediter. 2021;37(2):1147-1153.
28.
Deng  Y, You  C, Liu  Y, Qin  J, Zhou  X-H.  Estimation of incubation period and generation time based on observed length-biased epidemic cohort with censoring for COVID-19 outbreak in China.   Biometrics. 2021;77(3):929-941. doi:10.1111/biom.13325PubMedGoogle ScholarCrossref
29.
Denis  F, Septans  AL, Le Goff  F, Jeanneau  S, Lescure  FX.  Analysis of COVID-19 transmission sources in France by self-assessment before and after the partial lockdown: observational study.   J Med Internet Res. 2021;23(5):e26932. doi:10.2196/26932PubMedGoogle ScholarCrossref
30.
Ding  K, Yi  B, Chen  Y,  et al.  Epidemic cluster of novel coronavirus disease 2019 in Ningbo city of Zhejiang province.  Article in Chinese.  Chin J Publ Health. 2020;36(04):498-502.Google Scholar
31.
Dong  XC, Li  JM, Bai  JY,  et al.  [Epidemiological characteristics of confirmed COVID-19 cases in Tianjin].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(5):638-641.PubMedGoogle Scholar
32.
Du  ZC, Gu  J, Li  JH,  et al.  [Estimating the distribution of COVID-19 incubation period by interval-censored data estimation method].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(7):1000-1003.PubMedGoogle Scholar
33.
Gao  Y, Hou  L, Li  Y,  et al.  Epidemiological investigation and analysis of 137 confirmed cases of COVID-19.  Article in Chinese.  Chinese Journal of Critical Care Medicine. 2020;40(03):232-236.Google Scholar
34.
Gao  Y, Ma  X, Bi  J,  et al.  Epidemiological and clinical differences of coronavirus disease 2019 patients with distinct viral exposure history.   Virulence. 2020;11(1):1015-1023. doi:10.1080/21505594.2020.1802870PubMedGoogle ScholarCrossref
35.
Grant  R, Charmet  T, Schaeffer  L,  et al.  Impact of SARS-CoV-2 Delta variant on incubation, transmission settings and vaccine effectiveness: results from a nationwide case-control study in France.   Lancet Reg Health Eur. 2022;13:100278-100278. doi:10.1016/j.lanepe.2021.100278PubMedGoogle ScholarCrossref
36.
Guo  C-X, He  L, Yin  J-Y,  et al.  Epidemiological and clinical features of pediatric COVID-19.   BMC Med. 2020;18(1):250-250. doi:10.1186/s12916-020-01719-2PubMedGoogle ScholarCrossref
37.
Han  T.  Outbreak investigation: transmission of COVID-19 started from a spa facility in a local community in Korea.   Epidemiol Health. 2020;42:e2020056-e2020056. doi:10.4178/epih.e2020056PubMedGoogle ScholarCrossref
38.
Han  T, Hua  L, He  S,  et al.  The epidemiological characteristics of cluster transmission of coronavirus disease 2019 (COVID-19): a multi-center study in Jiangsu Province.   Am J Transl Res. 2020;12(10):6434-6444.PubMedGoogle Scholar
39.
Han  YN, Feng  ZW, Sun  LN,  et al.  A comparative-descriptive analysis of clinical characteristics in 2019-coronavirus-infected children and adults.   J Med Virol. 2020;92(9):1596-1602. doi:10.1002/jmv.25835PubMedGoogle ScholarCrossref
40.
Hong  H, Shi  HB, Jiang  HB,  et al.  [Epidemic dynamic model based evaluation of effectiveness of prevention and control strategies for COVID-19 in Ningbo].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(10):1606-1610.PubMedGoogle Scholar
41.
Hua  C-Z, Miao  Z-P, Zheng  J-S,  et al.  Epidemiological features and viral shedding in children with SARS-CoV-2 infection.   J Med Virol. 2020;92(11):2804-2812. doi:10.1002/jmv.26180PubMedGoogle Scholar
42.
Huang  J, Zhao  Q, Xu  K,  et al.  Analysis of COVID-19 epidemic characteristics based on network data.  Article in Chinese.  Chinese Journal of Disease Control & Prevention. 2020;24(11):1338.Google Scholar
43.
Huang  L, Zhang  X, Xu  A.  Effectiveness of interventions as part of the One Health approach to control coronavirus disease 2019 and stratified case features in Anhui Province, China: a real-world population-based cohort study.   One Health. 2021;12:100224-100224. doi:10.1016/j.onehlt.2021.100224PubMedGoogle Scholar
44.
Huang  S, Li  J, Dai  C,  et al.  Incubation period of coronavirus disease 2019: new implications for intervention and control.   Int J Environ Health Res. 2021;32(8):1707-1715.PubMedGoogle Scholar
45.
Je  D, O’Brolchain  A, Ulett  KB,  et al.  Demographics, clinical characteristics and outcomes among 197 patients with COVID-19 in the Gold Coast area.   Intern Med J. 2021;51(5):666-672. doi:10.1111/imj.15260PubMedGoogle Scholar
46.
Jeong  TH, Pak  C, Ock  M, Lee  SH, Son  JS, Jeon  YJ.  Real asymptomatic SARS-CoV-2 infection might be rare: importance of careful interviews and follow-up.   J Korean Med Sci. 2020;35(37):e333-e333. doi:10.3346/jkms.2020.35.e333PubMedGoogle Scholar
47.
Jiang  G, Wang  C, Song  L,  et al.  Aerosol transmission, an indispensable route of COVID-19 spread: case study of a department-store cluster.   Front Environ Sci Eng. 2021;15(3):46. doi:10.1007/s11783-021-1386-6PubMedGoogle Scholar
48.
Jiang  Z, Yang  B, Qin  J, Zhou  Y.  Enhanced empirical likelihood estimation of incubation period of COVID-19 by integrating published information.   Stat Med. 2021;40(19):4252-4268. doi:10.1002/sim.9026PubMedGoogle Scholar
49.
Jin  X, Lian  J-S, Hu  J-H,  et al.  Epidemiological, clinical and virological characteristics of 74 cases of coronavirus-infected disease 2019 (COVID-19) with gastrointestinal symptoms.   Gut. 2020;69(6):1002-1009. doi:10.1136/gutjnl-2020-320926PubMedGoogle Scholar
50.
Khonyongwa  K, Taori  SK, Soares  A,  et al.  Incidence and outcomes of healthcare-associated COVID-19 infections: significance of delayed diagnosis and correlation with staff absence.   J Hosp Infect. 2020;106(4):663-672. doi:10.1016/j.jhin.2020.10.006PubMedGoogle Scholar
51.
Ki  HJ, Kim  J, Kim  S, Park  J, Lee  J, Kim  Y-J.  Statistical analysis of estimating incubation period distribution and case fatality rate of COVID-19.   Korean Journal of Applied Statistics. 2020;33(6):777-789.Google Scholar
52.
Kim  SE, Jeong  HS, Yu  Y,  et al.  Viral kinetics of SARS-CoV-2 in asymptomatic carriers and presymptomatic patients.   Int J Infect Dis. 2020;95:441-443. doi:10.1016/j.ijid.2020.04.083Google Scholar
53.
Kong  D, Zheng  Y, Wu  H,  et al.  Pre-symptomatic transmission of novel coronavirus in community settings.   Influenza Other Respir Viruses. 2020;14(6):610-614. doi:10.1111/irv.12773PubMedGoogle Scholar
54.
Kong  T-K.  Longer incubation period of coronavirus disease 2019 (COVID-19) in older adults.   Aging Med (Milton). 2020;3(2):102-109. doi:10.1002/agm2.12114PubMedGoogle Scholar
55.
Kong  W, Wang  Y, Hu  J, Chughtai  A, Pu  H; Clinical Research Collaborative Group of Sichuan Provincial People’s Hospital.  Comparison of clinical and epidemiological characteristics of asymptomatic and symptomatic SARS-CoV-2 infection: a multi-center study in Sichuan Province, China.   Travel Med Infect Dis. 2020;37:101754-101754. doi:10.1016/j.tmaid.2020.101754PubMedGoogle Scholar
56.
Lai  C, Yu  R, Wang  M,  et al.  Shorter incubation period is associated with severe disease progression in patients with COVID-19.   Virulence. 2020;11(1):1443-1452. doi:10.1080/21505594.2020.1836894PubMedGoogle Scholar
57.
Lau  YC, Tsang  TK, Kennedy-Shaffer  L,  et al.  Joint estimation of generation time and incubation period for coronavirus disease (Covid-19).   J Infect Dis. 2021;224(10):1664-1671. doi:10.1093/infdis/jiab424PubMedGoogle Scholar
58.
Le  TQM, Takemura  T, Moi  ML,  et al.  Severe acute respiratory syndrome coronavirus 2 shedding by travelers, Vietnam, 2020.   Emerg Infect Dis. 2020;26(7):1624-1626. doi:10.3201/eid2607.200591PubMedGoogle Scholar
59.
Lee  JJ, Choe  YJ, Jeong  H,  et al.  Importation and transmission of SARS-CoV-2 B.1.1.529 (Omicron) variant of concern in Korea, November 2021.   J Korean Med Sci. 2021;36(50):e346-e346. doi:10.3346/jkms.2021.36.e346PubMedGoogle Scholar
60.
Lei  D, Wang  C, Li  C,  et al.  Clinical characteristics of COVID-19 in pregnancy: analysis of nine cases.  Article in Chinese. Chinese Journal of Perinatal Medicine. 2020;12:222-228.
61.
Leung  C.  The difference in the incubation period of 2019 novel coronavirus (SARS-CoV-2) infection between travelers to Hubei and nontravelers: the need for a longer quarantine period.   Infect Control Hosp Epidemiol. 2020;41(5):594-596. doi:10.1017/ice.2020.81PubMedGoogle Scholar
62.
Li  J, Ding  J, Chen  L,  et al.  Epidemiological and clinical characteristics of three family clusters of COVID-19 transmitted by latent patients in China.   Epidemiol Infect. 2020;148:e137-e137. doi:10.1017/S0950268820001491PubMedGoogle Scholar
63.
Li  L, Han  Z-G, Qin  P-Z,  et al.  Transmission and containment of the SARS-CoV-2 Delta variant of concern in Guangzhou, China: a population-based study.   PLoS Negl Trop Dis. 2022;16(1):e0010048-e0010048. doi:10.1371/journal.pntd.0010048PubMedGoogle Scholar
64.
Li  Q, Guan  X, Wu  P,  et al.  Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia.   N Engl J Med. 2020;382(13):1199-1207. doi:10.1056/NEJMoa2001316PubMedGoogle Scholar
65.
Linton  NM, Kobayashi  T, Yang  Y,  et al.  Incubation period and other epidemiological characteristics of 2019 novel coronavirus infections with right truncation: a statistical analysis of publicly available case data.   J Clin Med. 2020;9(2):538. doi:10.3390/jcm9020538PubMedGoogle Scholar
66.
Liu  C-X, Liu  Z-H, Sun  L-Y, Zhang  K-Y, Sun  Y-Z.  A familial cluster of COVID-19 infection in a northern Chinese region.   J Infect Public Health. 2021;14(9):1127-1132. doi:10.1016/j.jiph.2021.07.011PubMedGoogle Scholar
67.
Liu  F, Ji  C, Luo  J,  et al.  Clinical characteristics and corticosteroids application of different clinical types in patients with corona virus disease 2019.   Sci Rep. 2020;10(1):13689-13689. doi:10.1038/s41598-020-70387-2PubMedGoogle Scholar
68.
Liu  J, Liao  X, Qian  S,  et al.  Community transmission of severe acute respiratory syndrome coronavirus 2, Shenzhen, China, 2020.   Emerg Infect Dis. 2020;26(6):1320-1323. doi:10.3201/eid2606.200239PubMedGoogle Scholar
69.
Liu  JY, Chen  TJ, Hwang  SJ.  Analysis of community-acquired COVID-19 cases in Taiwan.   J Chin Med Assoc. 2020;83(12):1087-1092. doi:10.1097/JCMA.0000000000000411PubMedGoogle Scholar
70.
Liu  L, Du  Y, Bai  J,  et al.  Analysis of early characteristics of patients with novel coronavirus infection in Kunming City, Yunnan Province, China.  Article in Chinese. Chinese Journal of Zoonoses. 2020;36(5):424-428.
71.
Liu  L, Jing  L, Li  Y,  et al.  Clustering of 2019 novel coronavirus disease cases in Liaoning province: reported data-base analysis.  Article in Chinese.  Chin J Publ Health. 2020;36(04):473-476. doi:10.11847/zgggws1128823Google Scholar
72.
Liu  P, Niu  R, Chen  J,  et al.  Epidemiological and clinical features in patients with coronavirus disease 2019 outside of Wuhan, China: special focus in asymptomatic patients.   PLoS Negl Trop Dis. 2021;15(3):e0009248-e0009248. doi:10.1371/journal.pntd.0009248PubMedGoogle Scholar
73.
Liu  W, Zhou  M, Yang  S,  et al.  Epidemiological characteristics of cluster cases of coronavirus disease 2019 (COVID-19) in China.   Acta Medicinae Universitatis Scientiae et Technologiae Huazhong. 2020;49(02):161-168.Google Scholar
74.
Liu  Y, Ding  N, Zhou  S,  et al.  Comparison of clinical characteristics between patients with coronavirus disease 2019 (COVID-19) who retested RT-PCR positive versus negative: a retrospective study of data from Nanjing.   J Thorac Dis. 2020;12(11):6435-6445. doi:10.21037/jtd.2020.04.17PubMedGoogle Scholar
75.
Llaque-Quiroz  P, Prudencio-Gamio  R, Echevarría-Lopez  S, Ccorahua-Paz  M, Ugas-Charcape  C.  Clinical and epidemiological characteristics of children with COVID-19 in a pediatric hospital in Peru.   Rev Peru Med Exp Salud Publica. 2020;37(4):689-693. doi:10.17843/rpmesp.2020.374.6198PubMedGoogle Scholar
76.
Mao  S, Huang  T, Yuan  H,  et al.  Epidemiological analysis of 67 local COVID-19 clusters in Sichuan Province, China.   BMC Public Health. 2020;20(1):1525-1525. doi:10.1186/s12889-020-09606-4PubMedGoogle Scholar
77.
Moazzami  B, Moezedin Javad  R, Samie  S,  et al.  Is Computed tomography necessary for the diagnosis of coronavirus disease (COVID-19) in all suspected patients? a case series.  J Kerman Univ Med Sci. 2021;2:187-193.
78.
Ng  T-C, Cheng  H-Y, Chang  H-H,  et al.  Comparison of estimated effectiveness of case-based and population-based interventions on COVID-19 containment in Taiwan.   JAMA Intern Med. 2021;181(7):913-921. doi:10.1001/jamainternmed.2021.1644PubMedGoogle Scholar
79.
Nie  X, Fan  L, Mu  G,  et al.  Epidemiological characteristics and incubation period of 7015 confirmed cases with coronavirus disease 2019 outside Hubei Province in China.   J Infect Dis. 2020;222(1):26-33. doi:10.1093/infdis/jiaa211PubMedGoogle Scholar
80.
Ogata  T, Tanaka  H, Irie  F, Hirayama  A, Takahashi  Y.  Shorter incubation period among unvaccinated delta variant coronavirus disease 2019 patients in Japan.   Int J Environ Res Public Health. 2022;19(3):1127. doi:10.3390/ijerph19031127PubMedGoogle Scholar
81.
Pak  D, Langohr  K, Ning  J, Cortés Martínez  J, Gómez Melis  G, Shen  Y.  Modeling the coronavirus disease 2019 incubation period: impact on quarantine policy.   Mathematics. 2020;8(9):1631. doi:10.3390/math8091631Google Scholar
82.
Pan  XX, Chen  Y, Wang  AH,  et al.  [Study on transmission dynamic of 15 clusters of COVID-2019 cases in Ningbo].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(12):2010-2014.PubMedGoogle Scholar
83.
Patrikar  SR, Kotwal  A, Bhatti  VK,  et al.  Incubation period and reproduction number for novel coronavirus 2019 (COVID-19) infections in India.   Asia Pac J Public Health. 2020;32(8):458-460. doi:10.1177/1010539520956427PubMedGoogle Scholar
84.
Paul  S, Lorin  E.  Distribution of incubation periods of COVID-19 in the Canadian context.   Sci Rep. 2021;11(1):12569-12569. doi:10.1038/s41598-021-91834-8PubMedGoogle Scholar
85.
Ping  K, Lei  M, Gou  Y,  et al.  Epidemiologic characteristics of COVID-19 in Guizhou Province, China.   J Infect Dev Ctries. 2021;15(3):389-397. doi:10.3855/jidc.12818PubMedGoogle Scholar
86.
Pongpirul  WA, Wiboonchutikul  S, Charoenpong  L,  et al.  Clinical course and potential predictive factors for pneumonia of adult patients with Coronavirus Disease 2019 (COVID-19): a retrospective observational analysis of 193 confirmed cases in Thailand.   PLoS Negl Trop Dis. 2020;14(10):e0008806-e0008806. doi:10.1371/journal.pntd.0008806PubMedGoogle Scholar
87.
Pung  R, Chiew  CJ, Young  BE,  et al; Singapore 2019 Novel Coronavirus Outbreak Research Team.  Investigation of three clusters of COVID-19 in Singapore: implications for surveillance and response measures.   Lancet. 2020;395(10229):1039-1046. doi:10.1016/S0140-6736(20)30528-6PubMedGoogle Scholar
88.
Qi  B, Peng  H, Shou  K,  et al.  Protecting healthcare professionals during the COVID-19 pandemic.   Biomed Res Int. 2020;2020:8469560-8469560. doi:10.1155/2020/8469560PubMedGoogle Scholar
89.
Qian  GQ, Yang  NB, Ding  F,  et al.  Epidemiologic and clinical characteristics of 91 hospitalized patients with COVID-19 in Zhejiang, China: a retrospective, multi-centre case series.   QJM. 2020;113(7):474-481. doi:10.1093/qjmed/hcaa089PubMedGoogle Scholar
90.
Qiu  C, Deng  Z, Xiao  Q,  et al.  Transmission and clinical characteristics of coronavirus disease 2019 in 104 outside-Wuhan patients, China.   J Med Virol. 2020;92(10):2027-2035. doi:10.1002/jmv.25975PubMedGoogle Scholar
91.
Ratovoson  R, Razafimahatratra  R, Randriamanantsoa  L,  et al.  Household transmission of COVID-19 among the earliest cases in Antananarivo, Madagascar.   Influenza Other Respir Viruses. 2022;16(1):48-55. doi:10.1111/irv.12896PubMedGoogle Scholar
92.
Ren  X, Li  Y, Yang  X,  et al.  Evidence for pre-symptomatic transmission of coronavirus disease 2019 (COVID-19) in China.   Influenza Other Respir Viruses. 2021;15(1):19-26. doi:10.1111/irv.12787PubMedGoogle Scholar
93.
Samrah  SM, Al-Mistarehi  A-H, Kewan  T,  et al.  Viral clearance course of COVID-19 outbreaks.   J Multidiscip Healthc. 2021;14:555-565. doi:10.2147/JMDH.S302891PubMedGoogle Scholar
94.
Sanche  S, Lin  YT, Xu  C, Romero-Severson  E, Hengartner  N, Ke  R.  High contagiousness and rapid spread of severe acute respiratory syndrome coronavirus 2.   Emerg Infect Dis. 2020;26(7):1470-1477. doi:10.3201/eid2607.200282PubMedGoogle Scholar
95.
Shen  Q, Guo  W, Guo  T,  et al.  Novel coronavirus infection in children outside of Wuhan, China.   Pediatr Pulmonol. 2020;55(6):1424-1429. doi:10.1002/ppul.24762PubMedGoogle Scholar
96.
Shen  Y, Xu  W, Li  C,  et al.  A cluster of novel coronavirus disease 2019 infections indicating person-to-person transmission among casual contacts from social gatherings: an outbreak case-contact investigation.   Open Forum Infect Dis. 2020;7(6):ofaa231. doi:10.1093/ofid/ofaa231Google Scholar
97.
Shi  J-C, Yu  Z-J, He  G-Q,  et al.  Epidemiological features of 105 patients infected with the COVID-19.   J Natl Med Assoc. 2021;113(2):212-217. doi:10.1016/j.jnma.2020.09.151PubMedGoogle Scholar
98.
Shi  P, Gao  Y, Shen  Y,  et al.  Characteristics and evaluation of the effectiveness of monitoring and control measures for the first 69 patients with COVID-19 from 18 January 2020 to 2 March in Wuxi, China.   Sustain Cities Soc. 2021;64:102559-102559. doi:10.1016/j.scs.2020.102559PubMedGoogle Scholar
99.
Shiel  E, Miyakis  S, Tennant  E,  et al.  Clinical characteristics and outcomes of COVID-19 in a low-prevalence, well resourced setting, Sydney, Australia.   Intern Med J. 2021;51(10):1605-1613. doi:10.1111/imj.15445PubMedGoogle Scholar
100.
Shu  Y, Deng  Z, Wang  H,  et al.  Epidemiological and clinical characteristics of 32 patients with COVID-19.   Can J Infect Control. 2020;19(08):679-686.Google Scholar
101.
Song  Q, Zhao  H, Fang  L, Liu  W, Zheng  C, Zhang  Y.  Study on assessing early epidemiological parameters of coronavirus disease epidemic in China.  Article in Chinese.  Chinese Journal of Epidemiology. 2020;41(4):461-465. doi:10.3760/cma.j.cn112338-20200205-00069Google Scholar
102.
Song  R, Han  B, Song  M,  et al.  Clinical and epidemiological features of COVID-19 family clusters in Beijing, China.   J Infect. 2020;81(2):e26-e30. doi:10.1016/j.jinf.2020.04.018PubMedGoogle Scholar
103.
Song  YS, Hao  YB, Liu  WW, Zhang  SS, Wang  P, Fan  TL.  Clinical features of 17 patients with 2019-nCoV.   Eur Rev Med Pharmacol Sci. 2020;24(20):10896-10901.PubMedGoogle Scholar
104.
Su  YJ, Kuo  KC, Wang  TW, Chang  CW.  Gender-based differences in COVID-19.   New Microbes New Infect. 2021;42:100905-100905. doi:10.1016/j.nmni.2021.100905PubMedGoogle Scholar
105.
Sugano  N, Ando  W, Fukushima  W.  Cluster of severe acute respiratory syndrome coronavirus 2 infections linked to music clubs in Osaka, Japan.   J Infect Dis. 2020;222(10):1635-1640. doi:10.1093/infdis/jiaa542PubMedGoogle Scholar
106.
Sun  B, Qi  Y, Chen  H,  et al.  Epidemiological profile of a cluster of COVID-19 caused by an imported case.  Article in Chinese. Chin J Prev Med. 2021;22(04):246-249.
107.
Sun  C, Zhang  XB, Dai  Y, Xu  XZ, Zhao  J.  [Clinical analysis of 150 cases of 2019 novel coronavirus infection in Nanyang City, Henan Province].  Article in Chinese.  Zhonghua Jie He He Hu Xi Za Zhi. 2020;43(6):503-508.PubMedGoogle Scholar
108.
Sun  L, Shen  L, Fan  J,  et al.  Clinical features of patients with coronavirus disease 2019 from a designated hospital in Beijing, China.   J Med Virol. 2020;92(10):2055-2066. doi:10.1002/jmv.25966PubMedGoogle Scholar
109.
Sun  YM, Liu  F, Cai  W,  et al.  [Transmission chains of clusters of COVID-19 associated with a market in Beijing].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2021;42(3):427-432.PubMedGoogle Scholar
110.
Sun  Y, Tian  L, Du  X, Wang  H, Li  Y, Wu  R.  Epidemiological and clinical characteristics of a familial cluster of COVID-19.   Epidemiol Infect. 2020;148:e145-e145. doi:10.1017/S0950268820001521PubMedGoogle Scholar
111.
Tan  WYT, Wong  LY, Leo  YS, Toh  MPHS.  Does incubation period of COVID-19 vary with age? a study of epidemiologically linked cases in Singapore.   Epidemiol Infect. 2020;148:e197-e197. doi:10.1017/S0950268820001995PubMedGoogle Scholar
112.
Tanaka  H, Ogata  T, Shibata  T,  et al.  Shorter incubation period among COVID-19 cases with the BA.1 Omicron variant.   Int J Environ Res Public Health. 2022;19(10):6330. doi:10.3390/ijerph19106330PubMedGoogle Scholar
113.
SARS-CoV-2 variant with lineage B.1.351 clusters investigation team.  Linked transmission chains of imported SARS-CoV-2 variant B.1.351 across mainland France, January 2021.   Euro Surveill. 2021;26(13):2100333. doi:10.2807/1560-7917.ES.2021.26.13.2100333Google Scholar
114.
Tian  S, Hu  N, Lou  J,  et al.  Characteristics of COVID-19 infection in Beijing.   J Infect. 2020;80(4):401-406. doi:10.1016/j.jinf.2020.02.018PubMedGoogle Scholar
115.
Tindale  LC, Stockdale  JE, Coombe  M,  et al.  Evidence for transmission of COVID-19 prior to symptom onset.   Elife. 2020;9:e57149. doi:10.7554/eLife.57149PubMedGoogle Scholar
116.
Viego  V, Geri  M, Castiglia  J, Jouglard  E.  Incubation period and serial interval of Covid-19 in a chain of infections in Bahia Blanca (Argentina).   Cien Saude Colet. 2020;25(9):3503-3510. doi:10.1590/1413-81232020259.20852020PubMedGoogle Scholar
117.
Wang  J, Wang  S, Yang  S,  et al.  Epidemiological characteristics of 17 coronavirus disease 2019 in Puyang, Henan.  Article in Chinese. Chinese Journal of Disease Control & Prevention. 2021;25(04):416-420.
118.
Wang  X, Gao  J, Wang  X, Hu  W, Liu  H.  Clinical and epidemiological characteristics of patients with COVID.   Can J Infect Control. 2020;19(03):223-226. doi:10.3389/fpubh.2020.00244Google Scholar
119.
Wang  X, Pan  Y, Zhang  D,  et al.  Basic epidemiological parameter values from data of real-world in mega-cities: the characteristics of COVID-19 in Beijing, China.   BMC Infect Dis. 2020;20(1):526-526. doi:10.1186/s12879-020-05251-9PubMedGoogle Scholar
120.
Wang  X, Zhou  Q, He  Y,  et al.  Nosocomial outbreak of COVID-19 pneumonia in Wuhan, China.   Eur Respir J. 2020;55(6):2000544. doi:10.1183/13993003.00544-2020PubMedGoogle Scholar
121.
Wang  Y, Liao  B, Guo  Y,  et al.  Clinical characteristics of patients infected with the novel 2019 coronavirus (SARS-Cov-2) in Guangzhou, China.   Open Forum Infect Dis. 2020;7(6):ofaa187. doi:10.1093/ofid/ofaa187PubMedGoogle Scholar
122.
Wei  X.  Analysis of clinical characteristics of 28 cases of COVID-19.  Article in Chinese.  Chinese Journal of Integrated Traditional and Western Medicine. 2021;28(3).Google Scholar
123.
Wei  Y, Zeng  W, Huang  X,  et al.  Clinical characteristics of 276 hospitalized patients with coronavirus disease 2019 in Zengdu District, Hubei Province: a single-center descriptive study.   BMC Infect Dis. 2020;20(1):549-549. doi:10.1186/s12879-020-05252-8PubMedGoogle Scholar
124.
Won  YS, Kim  J-H, Ahn  CY, Lee  H.  Subcritical transmission in the early stage of COVID-19 in Korea.   Int J Environ Res Public Health. 2021;18(3):1265. doi:10.3390/ijerph18031265PubMedGoogle Scholar
125.
Wong  J, Chaw  L, Koh  WC,  et al.  Epidemiological investigation of the first 135 COVID-19 cases in Brunei: implications for surveillance, control, and travel restrictions.   Am J Trop Med Hyg. 2020;103(4):1608-1613. doi:10.4269/ajtmh.20-0771PubMedGoogle Scholar
126.
Wu  J, Chen  X, Gong  L,  et al.  Epidemiological and clinical features of SARS-CoV-2 cluster infection in Anhui Province, Eastern China.   Int J Infect Dis. 2021;117:372-377. doi:10.1016/j.ijid.2021.04.064Google Scholar
127.
Wu  J, Xia  J, Yang  Q,  et al.  Clinical characteristics of SARS-CoV-2 infection at early stage.  Article in Chinese.  Chinese Journal of Infection and Chemotherapy. 2020;20(05):521-524.Google Scholar
128.
Wu  W, Liu  Y, Wei  Z,  et al.  Investigation and analysis on characteristics of a cluster of COVID-19 associated with exposure in a department store in Tianjin.  Article in Chinese.  Chin J Epidemiol. 2020;41(4):489-493. doi:10.3760/cma.j.cn112338-20200221-00139Google Scholar
129.
Wu  Y.  Epidemiological and clinical characteristics of 41 patients with COVID-19.  Article in Chinese.  Chinese Journal of Integrated Traditional and Western Medicine. 2020;27(4).Google Scholar
130.
Xiao  F, Chen  B, Xiao  T, Lee  SK, Yan  K, Hu  L.  Children with SARS-CoV-2 infection during an epidemic in China (outside of Hubei province).   Ann Transl Med. 2020;8(14):849-849. doi:10.21037/atm-20-2908PubMedGoogle Scholar
131.
Xiao  M, Hou  M, Liu  X, Li  Z, Zhao  Q.  Clinical characteristics of 71 patients with coronavirus disease 2019.  Article in Chinese.  Zhong Nan Da Xue Xue Bao Yi Xue Ban. 2020;45(7):790-796. doi:10.11817/j.issn.1672-7347.2020.200187PubMedGoogle Scholar
132.
Xiao  Z, Guo  W, Luo  Z, Liao  J, Wen  F, Lin  Y.  Examining geographical disparities in the incubation period of the COVID-19 infected cases in Shenzhen and Hefei, China.   Environ Health Prev Med. 2021;26(1):10-10. doi:10.1186/s12199-021-00935-3PubMedGoogle Scholar
133.
Xiao  Z, Xie  X, Guo  W,  et al.  Examining the incubation period distributions of COVID-19 on Chinese patients with different travel histories.   J Infect Dev Ctries. 2020;14(4):323-327. doi:10.3855/jidc.12718PubMedGoogle Scholar
134.
Xie  S, Zhang  G, Yu  H,  et al.  The epidemiologic and clinical features of suspected and confirmed cases of imported 2019 novel coronavirus pneumonia in north Shanghai, China.   Ann Transl Med. 2020;8(10):637-637. doi:10.21037/atm-20-2119PubMedGoogle Scholar
135.
Xin  H, Li  Y, Wu  P,  et al.  Estimating the latent period of coronavirus disease 2019 (COVID-19).   Clin Infect Dis. 2022;74(9):1678-1681. doi:10.1093/cid/ciab746Google Scholar
136.
Xu  T, Chen  C, Zhu  Z,  et al.  Clinical features and dynamics of viral load in imported and non-imported patients with COVID-19.   Int J Infect Dis. 2020;94:68-71. doi:10.1016/j.ijid.2020.03.022Google Scholar
137.
Xu  X-W, Wu  X-X, Jiang  X-G,  et al.  Clinical findings in a group of patients infected with the 2019 novel coronavirus (SARS-Cov-2) outside of Wuhan, China: retrospective case series.   BMJ. 2020;368:m606-m606. doi:10.1136/bmj.m606PubMedGoogle Scholar
138.
Yang  HY, Xu  J, Li  Y,  et al.  [The preliminary analysis on the characteristics of the cluster for the COVID-19].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(5):623-628.PubMedGoogle Scholar
139.
Yang  J, Wu  K, Ding  A,  et al.  Clinical characteristics, treatment, and prognosis of 74 2019 novel coronavirus disease patients in Hefei: a single-center retrospective study.   Medicine (Baltimore). 2021;100(21):e25645-e25645. doi:10.1097/MD.0000000000025645PubMedGoogle Scholar
140.
Yang  N, Shen  Y, Shi  C,  et al.  In-flight transmission cluster of COVID-19: a retrospective case series.   Infect Dis (Lond). 2020;52(12):891-901. doi:10.1080/23744235.2020.1800814PubMedGoogle Scholar
141.
You  C, Deng  Y, Hu  W,  et al.  Estimation of the time-varying reproduction number of COVID-19 outbreak in China.   Int J Hyg Environ Health. 2020;228:113555-113555. doi:10.1016/j.ijheh.2020.113555PubMedGoogle Scholar
142.
Yu  S, Cui  S, Rui  J,  et al.  Epidemiological characteristics and transmissibility for SARS-CoV-2 of population level and cluster level in a Chinese city.   Front Public Health. 2022;9:799536-799536. doi:10.3389/fpubh.2021.799536PubMedGoogle Scholar
143.
Yu  X, Sun  X, Cui  P,  et al.  Epidemiological and clinical characteristics of 333 confirmed cases with coronavirus disease 2019 in Shanghai, China.   Transbound Emerg Dis. 2020;67(4):1697-1707. doi:10.1111/tbed.13604PubMedGoogle Scholar
144.
Zhang  H, Song  S, Chen  Z,  et al.  A Cluster transmission of coronavirus disease 2019 and the prevention and control measures in the early stage of the epidemic in Xi’an, China, 2020.   Med Sci Monit. 2021;27:e929701-e929701. doi:10.12659/MSM.929701PubMedGoogle Scholar
145.
Zhang  J, Litvinova  M, Wang  W,  et al.  Evolving epidemiology and transmission dynamics of coronavirus disease 2019 outside Hubei province, China: a descriptive and modelling study.   Lancet Infect Dis. 2020;20(7):793-802. doi:10.1016/S1473-3099(20)30230-9PubMedGoogle Scholar
146.
Zhang  K, Zhang  Y, Li  M, Yu  M, Cha  L, Ruan  L.  Comparative analysis of clinical characteristics of imported and local family clusters of novel coronavirus pneumonia.  Article in Chinese.  Chinese Journal of Integrated Traditional and Western Medicine. 2020;27(4):385-389.Google Scholar
147.
Zhang  Q, Zhu  J, Jia  C, Xu  S, Jiang  T, Wang  S.  Epidemiology and clinical outcomes of COVID-19 patients in northwestern China who had a history of exposure in Wuhan City: departure time-originated pinpoint surveillance.   Front Med (Lausanne). 2021;8:582299-582299. doi:10.3389/fmed.2021.582299PubMedGoogle Scholar
148.
Zhang  T, Ding  S, Zeng  Z,  et al.  Estimation of incubation period and serial interval for SARS-CoV-2 in Jiangxi, China, and an updated meta-analysis.   J Infect Dev Ctries. 2021;15(3):326-332. doi:10.3855/jidc.14025PubMedGoogle Scholar
149.
Zhao  C, Xu  Y, Zhang  X,  et al.  Public health initiatives from hospitalized patients with COVID-19, China.   J Infect Public Health. 2020;13(9):1229-1236. doi:10.1016/j.jiph.2020.06.013PubMedGoogle Scholar
150.
Zhao  H, Zhang  Z, Lun  W,  et al.  Transmission dynamics and successful control measures of SARS-CoV-2 in the mega-size city of Guangzhou, China.   Medicine (Baltimore). 2021;100(48):e27846. doi:10.1097/MD.0000000000027846PubMedGoogle Scholar
151.
Zhao  S, Tang  B, Musa  SS,  et al.  Estimating the generation interval and inferring the latent period of COVID-19 from the contact tracing data.   Epidemics. 2021;36:100482-100482. doi:10.1016/j.epidem.2021.100482PubMedGoogle Scholar
152.
Zhao  WH, Ma  Y, Wang  H,  et al.  [Epidemiological characteristics of three local epidemics of COVID-19 in Guangzhou].  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2021;42(12):2088-2095.PubMedGoogle Scholar
153.
Zhao  Z, Yin  M, Yin  S,  et al.  Analysis of clinical characteristics of 26 cases of COVID-19.  Article in Chinese.  Chinese Journal of Disease Control & Prevention. 2020;24(05):539-542.Google Scholar
154.
Zhong  S, Lin  F, Shi  L.  Clinical characteristics and outcomes of the patients with COVID-19: a report of 62 cases.  Article in Chinese.  Medical Journal of Chinese People’s Liberation Army. 2020;45(04):370-374.Google Scholar
155.
Zhu  J, Zhang  Q, Jia  C,  et al.  Challenges caused by imported cases abroad for the prevention and control of COVID-19 in China.   Front Med (Lausanne). 2021;8:573726-573726. doi:10.3389/fmed.2021.573726PubMedGoogle Scholar
156.
Zhu  N, Chen  S, Yang  G,  et al.  Analysis of the path from discovery of novel coronavirus disease 2019 cases to its diagnosis in Shaanxi Province.  Article in Chinese. Xi’an Jiaotong Daxue Xuebao Yixue Ban. 2020;41(06):923-926.
157.
Zhu  W, Zhang  M, Pan  J, Yao  Y, Wang  W.  Effects of prolonged incubation period and centralized quarantine on the COVID-19 outbreak in Shijiazhuang, China: a modeling study.   BMC Med. 2021;19(1):308-308. doi:10.1186/s12916-021-02178-zPubMedGoogle Scholar
158.
Wang  Y, Liao  B, Guo  Y,  et al.  Clinical characteristics of patients infected with the novel 2019 coronavirus (SARS-Cov-2) in Guangzhou, China.   Open Forum Infect Dis. 2020;7(6):ofaa187. doi:10.1093/ofid/ofaa187Google Scholar
159.
Elias  C, Sekri  A, Leblanc  P, Cucherat  M, Vanhems  P.  The incubation period of COVID-19: a meta-analysis.   Int J Infect Dis. 2021;104:708-710. doi:10.1016/j.ijid.2021.01.069Google Scholar
160.
Lessler  J, Reich  NG, Brookmeyer  R, Perl  TM, Nelson  KE, Cummings  DA.  Incubation periods of acute respiratory viral infections: a systematic review.   Lancet Infect Dis. 2009;9(5):291-300. doi:10.1016/S1473-3099(09)70069-6PubMedGoogle Scholar
161.
World Health Organization. Coronavirus disease (COVID-19). Updated May 13, 2021. Accessed April 17, 2022. https://www.who.int/emergencies/diseases/novel-coronavirus-2019/question-and-answers-hub/q-a-detail/coronavirus-disease-covid-19
162.
Cao  Y, Wang  J, Jian  F,  et al.  Omicron escapes the majority of existing SARS-CoV-2 neutralizing antibodies.   Nature. 2022;602(7898):657-663. doi:10.1038/s41586-021-04385-3PubMedGoogle Scholar
163.
US Centers for Disease Control and Prevention. Quarantine and Isolation. Updated March 30, 2022. Accessed April 17, 2022. https://www.cdc.gov/coronavirus/2019-ncov/your-health/quarantine-isolation.html
164.
Cowling  BJ, Muller  MP, Wong  IO,  et al.  Alternative methods of estimating an incubation distribution: examples from severe acute respiratory syndrome.   Epidemiology. 2007;18(2):253-259. doi:10.1097/01.ede.0000254660.07942.fbPubMedGoogle Scholar
165.
Chen  Y, Klein  SL, Garibaldi  BT,  et al.  Aging in COVID-19: vulnerability, immunity and intervention.   Ageing Res Rev. 2021;65:101205-101205. doi:10.1016/j.arr.2020.101205PubMedGoogle Scholar
166.
Virlogeux  V, Fang  VJ, Wu  JT,  et al.  Brief report: incubation period duration and severity of clinical disease following severe acute respiratory syndrome coronavirus infection.   Epidemiology. 2015;26(5):666-669. doi:10.1097/EDE.0000000000000339PubMedGoogle Scholar
167.
Virlogeux  V, Park  M, Wu  JT, Cowling  BJ.  Association between severity of MERS-CoV infection and incubation period.   Emerg Infect Dis. 2016;22(3):526-528. doi:10.3201/eid2203.151437PubMedGoogle Scholar
×