Global Percentage of Asymptomatic SARS-CoV-2 Infections Among the Tested Population and Individuals With Confirmed COVID-19 Diagnosis: A Systematic Review and Meta-analysis | Global Health | JAMA Network Open | JAMA Network
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Figure 1.  Flow Diagram of Study Selection
Flow Diagram of Study Selection
Figure 2.  Percentage of Asymptomatic Infections Among the Tested Population by Subgroups
Percentage of Asymptomatic Infections Among the Tested Population by Subgroups

Includes 29 776 306 tested individuals, among whom 11 516 had asymptomatic infections.

Figure 3.  Percentage of Asymptomatic Infections Among the Confirmed Population by Subgroups
Percentage of Asymptomatic Infections Among the Confirmed Population by Subgroups

Includes 19 884 individuals with confirmed COVID-19, among whom 11 069 had asymptomatic infections.

Figure 4.  Funnel Plots Based on the Percentage of Asymptomatic Infections
Funnel Plots Based on the Percentage of Asymptomatic Infections

Includes 29 776 306 tested individuals, among whom 11 516 had asymptomatic infections and 19 884 individuals with confirmed COVID-19, among whom 11 069 had asymptomatic infections. Funnel plot asymmetry indicated possible publication bias.

Table.  Characteristics of the Studies Included for Meta-analysis
Characteristics of the Studies Included for Meta-analysis
1.
Huang  C, Wang  Y, Li  X,  et al.  Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China.   Lancet. 2020;395(10223):497-506. doi:10.1016/S0140-6736(20)30183-5 PubMedGoogle ScholarCrossref
2.
World Health Organization. WHO coronavirus disease (COVID-19) dashboard. Accessed January 28, 2021. https://covid19.who.int/
3.
Kronbichler  A, Kresse  D, Yoon  S, Lee  KH, Effenberger  M, Shin  JI.  Asymptomatic patients as a source of COVID-19 infections: a systematic review and meta-analysis.   Int J Infect Dis. 2020;98:180-186. doi:10.1016/j.ijid.2020.06.052 PubMedGoogle ScholarCrossref
4.
World Health Organization. Coronavirus disease (COVID-19): how is it transmitted? December 13, 2020. Accessed January 22, 2021. https://www.who.int/news-room/q-a-detail/coronavirus-disease-covid-19-how-is-it-transmitted
5.
Chen  C, Zhu  C, Yan  D,  et al.  The epidemiological and radiographical characteristics of asymptomatic infections with the novel coronavirus (COVID-19): a systematic review and meta-analysis.   Int J Infect Dis. 2021;104:458-464. doi:10.1016/j.ijid.2021.01.017 PubMedGoogle ScholarCrossref
6.
Wilder-Smith  A, Chiew  CJ, Lee  VJ.  Can we contain the COVID-19 outbreak with the same measures as for SARS?   Lancet Infect Dis. 2020;20(5):e102-e107. doi:10.1016/S1473-3099(20)30129-8 PubMedGoogle ScholarCrossref
7.
Krishnasamy  N, Natarajan  M, Ramachandran  A,  et al.  Clinical outcomes among asymptomatic or mildly symptomatic COVID-19 patients in an isolation facility in Chennai, India.   Am J Trop Med Hyg. 2021;104(1):85-90. doi:10.4269/ajtmh.20-1096 PubMedGoogle ScholarCrossref
8.
Ra  SH, Lim  JS, Kim  GU, Kim  MJ, Jung  J, Kim  SH.  Upper respiratory viral load in asymptomatic individuals and mildly symptomatic patients with SARS-CoV-2 infection.   Thorax. 2021;76(1):61-63. doi:10.1136/thoraxjnl-2020-215042 PubMedGoogle ScholarCrossref
9.
He  X, Lau  EHY, Wu  P,  et al.  Temporal dynamics in viral shedding and transmissibility of COVID-19.   Nat Med. 2020;26(5):672-675. doi:10.1038/s41591-020-0869-5 PubMedGoogle ScholarCrossref
10.
Bai  Y, Yao  L, Wei  T,  et al.  Presumed asymptomatic carrier transmission of COVID-19.   JAMA. 2020;323(14):1406-1407. doi:10.1001/jama.2020.2565 PubMedGoogle ScholarCrossref
11.
Tong  ZD, Tang  A, Li  KF,  et al.  Potential presymptomatic transmission of SARS-CoV-2, Zhejiang Province, China, 2020.   Emerg Infect Dis. 2020;26(5):1052-1054. doi:10.3201/eid2605.200198 PubMedGoogle ScholarCrossref
12.
Arons  MM, Hatfield  KM, Reddy  SC,  et al; Public Health–Seattle and King County and CDC COVID-19 Investigation Team.  Presymptomatic SARS-CoV-2 infections and transmission in a skilled nursing facility.   N Engl J Med. 2020;382(22):2081-2090. doi:10.1056/NEJMoa2008457 PubMedGoogle ScholarCrossref
13.
Zhang  Y, Muscatello  D, Tian  Y,  et al.  Role of presymptomatic transmission of COVID-19: evidence from Beijing, China.   J Epidemiol Community Health. 2021;75(1):84-87. doi:10.1136/jech-2020-214635PubMedGoogle Scholar
14.
He  J, Guo  Y, Mao  R, Zhang  J.  Proportion of asymptomatic coronavirus disease 2019: a systematic review and meta-analysis.   J Med Virol. 2021;93(2):820-830. doi:10.1002/jmv.26326 PubMedGoogle ScholarCrossref
15.
Moola  S, Munn  Z, Tufanaru  C,  et al. Chapter 7: Systematic reviews of etiology and risk. In: Aromataris E, Munn Z, eds. JBI Manual for Evidence Synthesis. JBI; 2020. Accessed September 5, 2021. https://jbi-global-wiki.refined.site/space/MANUAL
16.
Wells  GA, Shea  B, O'Connell  D, Peterson  J, Welch  V, Losos  M, Tugwell  P.  The Newcastle-Ottawa Scale (NOS) for Assessing the Quality of Nonrandomised Studies in Meta-analyses. Ottawa Hospital Research Institute; 2011. Accessed September 5, 2021. http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp
17.
Yanes-Lane  M, Winters  N, Fregonese  F,  et al.  Proportion of asymptomatic infection among COVID-19 positive persons and their transmission potential: a systematic review and meta-analysis.   PLoS One. 2020;15(11):e0241536. doi:10.1371/journal.pone.0241536 PubMedGoogle Scholar
18.
DerSimonian  R, Laird  N.  Meta-analysis in clinical trials revisited.   Contemp Clin Trials. 2015;45(Pt A):139-145. doi:10.1016/j.cct.2015.09.002PubMedGoogle Scholar
19.
Higgins  JP, Thompson  SG, Deeks  JJ, Altman  DG.  Measuring inconsistency in meta-analyses.   BMJ. 2003;327(7414):557-560. doi:10.1136/bmj.327.7414.557 PubMedGoogle ScholarCrossref
20.
Egger  M, Davey Smith  G, Schneider  M, Minder  C.  Bias in meta-analysis detected by a simple, graphical test.   BMJ. 1997;315(7109):629-634. doi:10.1136/bmj.315.7109.629 PubMedGoogle ScholarCrossref
21.
Knapp  G, Hartung  J.  Improved tests for a random effects meta-regression with a single covariate.   Stat Med. 2003;22(17):2693-2710. doi:10.1002/sim.1482 PubMedGoogle ScholarCrossref
22.
Abdelmoniem  R, Fouad  R, Shawky  S,  et al.  SARS-CoV-2 infection among asymptomatic healthcare workers of the emergency department in a tertiary care facility.   J Clin Virol. 2021;134:104710. doi:10.1016/j.jcv.2020.104710 PubMedGoogle Scholar
23.
Abeysuriya  S, Wasif  S, Counihan  C,  et al.  Universal screening for SARS-CoV-2 in pregnant women at term admitted to an East London maternity unit.   Eur J Obstet Gynecol Reprod Biol. 2020;252:444-446. doi:10.1016/j.ejogrb.2020.07.035 PubMedGoogle ScholarCrossref
24.
Akbarialiabad  H, Abdolrahimzadeh Fard  H, Abbasi  HR,  et al.  Our experience of trauma management during novel coronovirus 2019 (COVID-19) pandemic in a busy trauma center in southern Iran.   Bull Emerg Trauma. 2020;8(3):199-201.PubMedGoogle Scholar
25.
Al-Qahtani  M, AlAli  S, AbdulRahman  A, Salman Alsayyad  A, Otoom  S, Atkin  SL.  The prevalence of asymptomatic and symptomatic COVID-19 in a cohort of quarantined subjects.   Int J Infect Dis. 2021;102:285-288. doi:10.1016/j.ijid.2020.10.091 PubMedGoogle ScholarCrossref
26.
Al-Shamsi  HO, Coomes  EA, Aldhaheri  K, Alrawi  S.  Serial screening for COVID-19 in asymptomatic patients receiving anticancer therapy in the United Arab Emirates.   JAMA Oncol. 2021;7(1):129-131. doi:10.1001/jamaoncol.2020.5745 PubMedGoogle ScholarCrossref
27.
Arnold  FW, Bishop  S, Oppy  L, Scott  L, Stevenson  G.  Surveillance testing reveals a significant proportion of hospitalized patients with SARS-CoV-2 are asymptomatic.   Am J Infect Control. 2021;49(3):281-285. doi:10.1016/j.ajic.2021.01.005 PubMedGoogle ScholarCrossref
28.
Aslam  A, Singh  J, Robilotti  E,  et al.  SARS CoV-2 surveillance and exposure in the perioperative setting with universal testing and personal protective equipment (PPE) policies.   Clin Infect Dis. Published online October 22, 2020. doi:10.1093/cid/ciaa1607 Google Scholar
29.
Bayle  C, Cantin  D, Vidal  JS,  et al; APHP COVID 19 research collaboration.  Asymptomatic SARS COV-2 carriers among nursing home staff: A source of contamination for residents?   Infect Dis Now. 2021;51(2):197-200. doi:10.1016/j.idnow.2020.11.008PubMedGoogle ScholarCrossref
30.
Bender  WR, Hirshberg  A, Coutifaris  P, Acker  AL, Srinivas  SK.  Universal testing for severe acute respiratory syndrome coronavirus 2 in 2 Philadelphia hospitals: carrier prevalence and symptom development over 2 weeks.   Am J Obstet Gynecol MFM. 2020;2(4):100226. doi:10.1016/j.ajogmf.2020.100226 PubMedGoogle Scholar
31.
Bianco  A, Buckley  AB, Overbey  J,  et al.  Testing of patients and support persons for coronavirus disease 2019 (COVID-19) infection before scheduled deliveries.   Obstet Gynecol. 2020;136(2):283-287. doi:10.1097/AOG.0000000000003985 PubMedGoogle ScholarCrossref
32.
Blain  H, Rolland  Y, Tuaillon  E,  et al.  Efficacy of a test-retest strategy in residents and health care personnel of a nursing home facing a COVID-19 outbreak.   J Am Med Dir Assoc. 2020;21(7):933-936. doi:10.1016/j.jamda.2020.06.013 PubMedGoogle ScholarCrossref
33.
Blitz  MJ, Rochelson  B, Rausch  AC,  et al.  Universal testing for coronavirus disease 2019 in pregnant women admitted for delivery: prevalence of peripartum infection and rate of asymptomatic carriers at four New York hospitals within an integrated healthcare system.   Am J Obstet Gynecol MFM. 2020;2(3):100169. doi:10.1016/j.ajogmf.2020.100169 PubMedGoogle Scholar
34.
Blumberg  TJ, Adler  AC, Lin  EE,  et al.  Universal screening for COVID-19 in children undergoing orthopaedic surgery: a multicenter report.   J Pediatr Orthop. 2020;40(10):e990-e993. doi:10.1097/BPO.0000000000001657 PubMedGoogle ScholarCrossref
35.
Bosworth  A, Whalley  C, Poxon  C,  et al.  Rapid implementation and validation of a cold-chain free SARS-CoV-2 diagnostic testing workflow to support surge capacity.   J Clin Virol. 2020;128:104469. doi:10.1016/j.jcv.2020.104469 PubMedGoogle Scholar
36.
Cao  S, Gan  Y, Wang  C,  et al.  Post-lockdown SARS-CoV-2 nucleic acid screening in nearly ten million residents of Wuhan, China.   Nat Commun. 2020;11(1):5917. doi:10.1038/s41467-020-19802-w PubMedGoogle ScholarCrossref
37.
Carroll  C, Conway  R, O’Donnell  D,  et al.  Routine testing of close contacts of confirmed COVID-19 cases: national COVID-19 contact management programme, Ireland, May to August 2020.   Public Health. 2021;190:147-151. PubMedGoogle ScholarCrossref
38.
Cattelan  AM, Sasset  L, Di Meco  E,  et al.  An integrated strategy for the prevention of SARS-CoV-2 infection in healthcare workers: a prospective observational study.   Int J Environ Res Public Health. 2020;17(16):E5785. doi:10.3390/ijerph17165785 PubMedGoogle Scholar
39.
Cloutier  L, Merindol  N, Pépin  G,  et al.  Asymptomatic carriers of COVID-19 in a confined adult community population in Quebec: a cross-sectional study.   Am J Infect Control. 2021;49(1):120-122. doi:10.1016/j.ajic.2020.08.015 PubMedGoogle ScholarCrossref
40.
Corcorran  MA, Olin  S, Rani  G,  et al.  Prolonged persistence of PCR-detectable virus during an outbreak of SARS-CoV-2 in an inpatient geriatric psychiatry unit in King County, Washington.   Am J Infect Control. 2021;49(3):293-298. PubMedGoogle ScholarCrossref
41.
Deng  ZQ, Xia  W, Fan  YB,  et al.  Analysis on transmission chain of a cluster epidemic of COVID-19, Nanchang [in Chinese].   Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(9):1420-1423.PubMedGoogle Scholar
42.
Dora  AV, Winnett  A, Jatt  LP,  et al.  Universal and serial laboratory testing for SARS-CoV-2 at a long-term care skilled nursing facility for veterans: Los Angeles, California, 2020.   MMWR Morb Mortal Wkly Rep. 2020;69(21):651-655. doi:10.15585/mmwr.mm6921e1 PubMedGoogle ScholarCrossref
43.
Duan  P, Deng  ZQ, Pan  ZY, Wang  YP.  Safety considerations during return to work in the context of stable COVID-19 epidemic control: an analysis of health screening results of all returned staff from a hospital.   Epidemiol Infect. 2020;148:e214. doi:10.1017/S0950268820002150 PubMedGoogle Scholar
44.
Figueiredo  R, Tavares  S, Moucho  M, Ramalho  C.  Systematic screening for SARS-CoV-2 in pregnant women admitted for delivery in a Portuguese maternity.   J Perinat Med. 2020;48(9):977-980. doi:10.1515/jpm-2020-0387 PubMedGoogle ScholarCrossref
45.
Goldfarb  IT, Diouf  K, Barth  WH,  et al.  Universal SARS-CoV-2 testing on admission to the labor and delivery unit: low prevalence among asymptomatic obstetric patients.   Infect Control Hosp Epidemiol. 2020;41(9):1095-1096. doi:10.1017/ice.2020.255 PubMedGoogle ScholarCrossref
46.
Graham  NSN, Junghans  C, Downes  R,  et al.  SARS-CoV-2 infection, clinical features and outcome of COVID-19 in United Kingdom nursing homes.   J Infect. 2020;81(3):411-419. doi:10.1016/j.jinf.2020.05.073 PubMedGoogle ScholarCrossref
47.
Grechukhina  O, Greenberg  V, Lundsberg  LS,  et al.  Coronavirus disease 2019 pregnancy outcomes in a racially and ethnically diverse population.   Am J Obstet Gynecol MFM. 2020;2(4)(suppl):100246. doi:10.1016/j.ajogmf.2020.100246 PubMedGoogle Scholar
48.
Gruskay  JA, Dvorzhinskiy  A, Konnaris  MA,  et al.  Universal testing for COVID-19 in essential orthopaedic surgery reveals a high percentage of asymptomatic infections.   J Bone Joint Surg Am. 2020;102(16):1379-1388. doi:10.2106/JBJS.20.01053 PubMedGoogle ScholarCrossref
49.
Han  X, Wei  X, Alwalid  O,  et al.  Severe acute respiratory syndrome coronavirus 2 among asymptomatic workers screened for work resumption, China.   Emerg Infect Dis. 2020;26(9):2265-2267. doi:10.3201/eid2609.201848 PubMedGoogle ScholarCrossref
50.
Harada  S, Uno  S, Ando  T,  et al.  Control of a nosocomial outbreak of COVID-19 in a university hospital.   Open Forum Infect Dis. 2020;7(12):a512. doi:10.1093/ofid/ofaa512PubMedGoogle ScholarCrossref
51.
Hcini  N, Maamri  F, Picone  O,  et al.  Maternal, fetal and neonatal outcomes of large series of SARS-CoV-2 positive pregnancies in peripartum period: a single-center prospective comparative study.   Eur J Obstet Gynecol Reprod Biol. 2021;257:11-18. doi:10.1016/j.ejogrb.2020.11.068 PubMedGoogle ScholarCrossref
52.
Hoxha  A, Wyndham-Thomas  C, Klamer  S,  et al.  Asymptomatic SARS-CoV-2 infection in Belgian long-term care facilities.   Lancet Infect Dis. 2021;21(4):e67. doi:10.1016/S1473-3099(20)30560-0PubMedGoogle Scholar
53.
Hung  IFN, Cheng  VCC, Li  X,  et al.  SARS-CoV-2 shedding and seroconversion among passengers quarantined after disembarking a cruise ship: a case series.   Lancet Infect Dis. 2020;20(9):1051-1060. doi:10.1016/S1473-3099(20)30364-9 PubMedGoogle ScholarCrossref
54.
Ibrahim  F, Natasha  A, Saharman  YR, Sudarmono  P.  Preliminary report of COVID-19 testing: experience of the clinical microbiology laboratory Universitas Indonesia, Jakarta, Indonesia.   New Microbes New Infect. 2020;37:100733. doi:10.1016/j.nmni.2020.100733 PubMedGoogle Scholar
55.
Kennelly  SP, Dyer  AH, Noonan  C,  et al.  Asymptomatic carriage rates and case-fatality of SARS-CoV-2 infection in residents and staff in Irish nursing homes.   Age Ageing. 2021;50(1):49-54. doi:10.1093/ageing/afaa220PubMedGoogle ScholarCrossref
56.
Kessler  T, Wiebe  J, Graf  T, Schunkert  H, Kastrati  A, Sager  HB.  SARS-CoV-2 infection in asymptomatic patients hospitalized for cardiac emergencies: implications for patient management.   Front Cardiovasc Med. 2020;7:599299. doi:10.3389/fcvm.2020.599299 PubMedGoogle Scholar
57.
Kimball  A, Hatfield  KM, Arons  M,  et al; Public Health–Seattle & King County; CDC COVID-19 Investigation Team.  Asymptomatic and presymptomatic SARS-CoV-2 infections in residents of a long-term care skilled nursing facility: King County, Washington, March 2020.   MMWR Morb Mortal Wkly Rep. 2020;69(13):377-381. doi:10.15585/mmwr.mm6913e1 PubMedGoogle ScholarCrossref
58.
Kirshblum  SC, DeLauter  G, Lopreiato  MC,  et al.  Screening testing for SARS-CoV-2 upon admission to rehabilitation hospitals in a high COVID-19 prevalence community.   PM R. 2020;12(10):1009-1014. doi:10.1002/pmrj.12454 PubMedGoogle ScholarCrossref
59.
Krüger  S, Leskien  M, Schuller  P,  et al.  Performance and feasibility of universal PCR admission screening for SARS-CoV-2 in a German tertiary care hospital.   J Med Virol. 2021;93(5):2890-2898. doi:10.1002/jmv.26770 PubMedGoogle ScholarCrossref
60.
Kwon  YS, Park  SH, Kim  HJ,  et al.  Screening clinic for coronavirus disease 2019 to prevent intrahospital spread in Daegu, Korea: a single-center report.   J Korean Med Sci. 2020;35(26):e246. doi:10.3346/jkms.2020.35.e246 PubMedGoogle Scholar
61.
LaCourse  SM, Kachikis  A, Blain  M,  et al.  Low prevalence of SARS-CoV-2 among pregnant and postpartum patients with universal screening in Seattle, Washington.   Clin Infect Dis. 2021;72(5):869-872. doi:10.1093/cid/ciaa675PubMedGoogle ScholarCrossref
62.
Ladhani  SN, Chow  JY, Janarthanan  R,  et al.  Investigation of SARS-CoV-2 outbreaks in six care homes in London, April 2020.   EClinicalMedicine. 2020;26:100533. doi:10.1016/j.eclinm.2020.100533 PubMedGoogle Scholar
63.
Lan  FY, Suharlim  C, Kales  SN, Yang  J.  Association between SARS-CoV-2 infection, exposure risk and mental health among a cohort of essential retail workers in the USA.   Occup Environ Med. 2021;78(4):237-243. PubMedGoogle Scholar
64.
Lavezzo  E, Franchin  E, Ciavarella  C,  et al; Imperial College COVID-19 Response Team; Imperial College COVID-19 Response Team.  Suppression of a SARS-CoV-2 outbreak in the Italian municipality of Vo’.   Nature. 2020;584(7821):425-429. doi:10.1038/s41586-020-2488-1 PubMedGoogle ScholarCrossref
65.
Livingston  G, Rostamipour  H, Gallagher  P,  et al.  Prevalence, management, and outcomes of SARS-CoV-2 infections in older people and those with dementia in mental health wards in London, UK: a retrospective observational study.   Lancet Psychiatry. 2020;7(12):1054-1063. doi:10.1016/S2215-0366(20)30434-X PubMedGoogle ScholarCrossref
66.
Lombardi  A, Consonni  D, Carugno  M,  et al.  Characteristics of 1573 healthcare workers who underwent nasopharyngeal swab testing for SARS-CoV-2 in Milan, Lombardy, Italy.   Clin Microbiol Infect. 2020;26(10):1413.e9-1413.e13. doi:10.1016/j.cmi.2020.06.013PubMedGoogle ScholarCrossref
67.
Ly  TDA, Zanini  D, Laforge  V,  et al.  Pattern of SARS-CoV-2 infection among dependant elderly residents living in long-term care facilities in Marseille, France, March-June 2020.   Int J Antimicrob Agents. 2020;56(6):106219. doi:10.1016/j.ijantimicag.2020.106219 PubMedGoogle Scholar
68.
Lytras  T, Dellis  G, Flountzi  A,  et al.  High prevalence of SARS-CoV-2 infection in repatriation flights to Greece from three European countries.   J Travel Med. 2020;27(3):taaa054. doi:10.1093/jtm/taaa054 PubMedGoogle Scholar
69.
Maechler  F, Gertler  M, Hermes  J,  et al.  Epidemiological and clinical characteristics of SARS-CoV-2 infections at a testing site in Berlin, Germany, March and April 2020-a cross-sectional study.   Clin Microbiol Infect. 2020;26(12):1685.e7-1685.e12. doi:10.1016/j.cmi.2020.08.017PubMedGoogle ScholarCrossref
70.
Marossy  A, Rakowicz  S, Bhan  A,  et al.  A study of universal severe acute respiratory syndrome coronavirus 2 RNA testing among residents and staff in a large group of care homes in South London.   J Infect Dis. 2021;223(3):381-388. doi:10.1093/infdis/jiaa565PubMedGoogle ScholarCrossref
71.
Marschner  S, Corradini  S, Rauch  J,  et al.  SARS-CoV-2 prevalence in an asymptomatic cancer cohort: results and consequences for clinical routine.   Radiat Oncol. 2020;15(1):165. doi:10.1186/s13014-020-01609-0 PubMedGoogle ScholarCrossref
72.
Martinez-Fierro  ML, Ríos-Jasso  J, Garza-Veloz  I,  et al.  The role of close contacts of COVID-19 patients in the SARS-CoV-2 transmission: an emphasis on the percentage of nonevaluated positivity in Mexico.   Am J Infect Control. 2021;49(1):15-20. doi:10.1016/j.ajic.2020.10.002 PubMedGoogle ScholarCrossref
73.
Massarotti  C, Adriano  M, Cagnacci  A,  et al.  Asymptomatic SARS-CoV-2 infections in pregnant patients in an Italian city during the complete lockdown.   J Med Virol. 2021;93(3):1758-1760. doi:10.1002/jmv.26458 PubMedGoogle ScholarCrossref
74.
Mattar  S, Martinez-Bravo  C, Rivero  R,  et al.  Epidemiological and viral features of a cohort of SARS-CoV-2 symptomatic and asymptomatic individuals in an area of the Colombian Caribbean.   Ann Clin Microbiol Antimicrob. 2020;19(1):58. doi:10.1186/s12941-020-00397-5 PubMedGoogle ScholarCrossref
75.
Menting  T, Krause  K, Benz-Tettey  F,  et al.  Low-threshold SARS-CoV-2 testing facility for hospital staff: prevention of COVID-19 outbreaks?   Int J Hyg Environ Health. 2021;231:113653. doi:10.1016/j.ijheh.2020.113653 PubMedGoogle Scholar
76.
Migueres  M, Mengelle  C, Dimeglio  C,  et al.  Saliva sampling for diagnosing SARS-CoV-2 infections in symptomatic patients and asymptomatic carriers.   J Clin Virol. 2020;130:104580. doi:10.1016/j.jcv.2020.104580 PubMedGoogle Scholar
77.
Milani  GP, Montomoli  E, Bollati  V,  et al; UNICORN Consortium investigators.  SARS-CoV-2 infection among asymptomatic homebound subjects in Milan, Italy.   Eur J Intern Med. 2020;78:161-163. doi:10.1016/j.ejim.2020.06.010 PubMedGoogle ScholarCrossref
78.
Nishiura  H, Kobayashi  T, Miyama  T,  et al.  Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19).   Int J Infect Dis. 2020;94:154-155. doi:10.1016/j.ijid.2020.03.020 PubMedGoogle ScholarCrossref
79.
Ochiai  D, Kasuga  Y, Iida  M, Ikenoue  S, Tanaka  M.  Universal screening for SARS-CoV-2 in asymptomatic obstetric patients in Tokyo, Japan.   Int J Gynaecol Obstet. 2020;150(2):268-269. doi:10.1002/ijgo.13252 PubMedGoogle ScholarCrossref
80.
Olalla  J, Correa  AM, Martín-Escalante  MD,  et al; ROBLE group.  Search for asymptomatic carriers of SARS-CoV-2 in healthcare workers during the pandemic: a Spanish experience.   QJM. 2020;hcaa238.PubMedGoogle Scholar
81.
Olmos  C, Campaña  G, Monreal  V,  et al.  SARS-CoV-2 infection in asymptomatic healthcare workers at a clinic in Chile.   PLoS One. 2021;16(1):e0245913. doi:10.1371/journal.pone.0245913 PubMedGoogle Scholar
82.
Park  SY, Kim  YM, Yi  S,  et al.  Coronavirus disease outbreak in call center, South Korea.   Emerg Infect Dis. 2020;26(8):1666-1670. doi:10.3201/eid2608.201274 PubMedGoogle ScholarCrossref
83.
Park  JH, Jang  JH, Lee  K, Yoo  SJ, Shin  H.  COVID-19 outbreak and presymptomatic transmission in pilgrim travelers who returned to Korea from Israel.   J Korean Med Sci. 2020;35(48):e424. doi:10.3346/jkms.2020.35.e424 PubMedGoogle Scholar
84.
Patel  MC, Chaisson  LH, Borgetti  S,  et al.  Asymptomatic SARS-CoV-2 infection and COVID-19 mortality during an outbreak investigation in a skilled nursing facility.   Clin Infect Dis. 2020;71(11):2920-2926. doi:10.1093/cid/ciaa763 PubMedGoogle ScholarCrossref
85.
Pavli  A, Smeti  P, Hadjianastasiou  S,  et al.  In-flight transmission of COVID-19 on flights to Greece: an epidemiological analysis.   Travel Med Infect Dis. 2020;38:101882. doi:10.1016/j.tmaid.2020.101882 PubMedGoogle Scholar
86.
Petersen  I, Phillips  A.  Three quarters of people with SARS-CoV-2 infection are asymptomatic: analysis of English household survey data.   Clin Epidemiol. 2020;12:1039-1043. doi:10.2147/CLEP.S276825 PubMedGoogle ScholarCrossref
87.
Puckett  Y, Wilke  L, Weber  S, Parkes  A, LoConte  NK.  Low rate of SARS-CoV-2 infection in adults with active cancer diagnosis in a nonendemic region in the United States.   WMJ. 2020;119(4):286-288.PubMedGoogle Scholar
88.
Ralli  M, Morrone  A, Arcangeli  A, Ercoli  L.  Asymptomatic patients as a source of transmission of COVID-19 in homeless shelters.   Int J Infect Dis. 2021;103:243-245. doi:10.1016/j.ijid.2020.12.031PubMedGoogle ScholarCrossref
89.
Rashid-Abdi  M, Krifors  A, Sälléber  A, Eriksson  J, Månsson  E.  Low rate of COVID-19 seroconversion in health-care workers at a Department of Infectious Diseases in Sweden during the later phase of the first wave: a prospective longitudinal seroepidemiological study.   Infect Dis (Lond). 2021;53(3):169-175. doi:10.1080/23744235.2020.1849787PubMedGoogle ScholarCrossref
90.
Ren  R, Zhang  Y, Li  Q,  et al.  Asymptomatic SARS-CoV-2 infections among persons entering China from April 16 to October 12, 2020.   JAMA. 2021;325(5):489-492. doi:10.1001/jama.2020.23942 PubMedGoogle ScholarCrossref
91.
Rincón  A, Moreso  F, López-Herradón  A,  et al.  The keys to control a COVID-19 outbreak in a haemodialysis unit.   Clin Kidney J. 2020;13(4):542-549. doi:10.1093/ckj/sfaa119 PubMedGoogle ScholarCrossref
92.
Roxby  AC, Greninger  AL, Hatfield  KM,  et al.  Outbreak investigation of COVID-19 among residents and staff of an independent and assisted living community for older adults in Seattle, Washington.   JAMA Intern Med. 2020;180(8):1101-1105. doi:10.1001/jamainternmed.2020.2233 PubMedGoogle ScholarCrossref
93.
Sacco  G, Foucault  G, Briere  O, Annweiler  C.  COVID-19 in seniors: findings and lessons from mass screening in a nursing home.   Maturitas. 2020;141:46-52. doi:10.1016/j.maturitas.2020.06.023 PubMedGoogle ScholarCrossref
94.
Santos  EJF, Ferreira  RJO, Batista  R,  et al.  Health care workers not in the frontline are more frequently carriers of coronavirus disease 2019: the experience of a tertiary Portuguese hospital.   Infect Prev Pract. 2020;2(4):100099. doi:10.1016/j.infpip.2020.100099 PubMedGoogle Scholar
95.
Scheier  T, Schibli  A, Eich  G,  et al.  Universal admission screening for SARS-CoV-2 infections among hospitalized patients, Switzerland, 2020.   Emerg Infect Dis. 2021;27(2):404-410. doi:10.3201/eid2702.202318 PubMedGoogle ScholarCrossref
96.
Shah  AS, Walkoff  LA, Kuzo  RS,  et al.  The utility of chest computed tomography (CT) and RT-PCR screening of asymptomatic patients for SARS-CoV-2 prior to semiurgent or urgent hospital procedures.   Infect Control Hosp Epidemiol. 2020;41(12):1375-1377. doi:10.1017/ice.2020.331 PubMedGoogle ScholarCrossref
97.
Shi  SM, Bakaev  I, Chen  H, Travison  TG, Berry  SD.  Risk factors, presentation, and course of coronavirus disease 2019 in a large, academic long-term care facility.   J Am Med Dir Assoc. 2020;21(10):1378-1383.e1. doi:10.1016/j.jamda.2020.08.027 PubMedGoogle ScholarCrossref
98.
Singer  JS, Cheng  EM, Murad  DA,  et al.  Low prevalence (0.13%) of COVID-19 infection in asymptomatic pre-operative/pre-procedure patients at a large, academic medical center informs approaches to perioperative care.   Surgery. 2020;168(6):980-986. doi:10.1016/j.surg.2020.07.048 PubMedGoogle ScholarCrossref
99.
Tang  H, Tian  JB, Dong  JW,  et al.  Serologic detection of SARS-CoV-2 infections in hemodialysis centers: a multicenter retrospective study in Wuhan, China.   Am J Kidney Dis. 2020;76(4):490-499.e1. doi:10.1053/j.ajkd.2020.06.008 PubMedGoogle ScholarCrossref
100.
Tang  O, Bigelow  BF, Sheikh  F,  et al.  Outcomes of nursing home COVID-19 patients by initial symptoms and comorbidity: results of universal testing of 1970 residents.   J Am Med Dir Assoc. 2020;21(12):1767-1773.e1. doi:10.1016/j.jamda.2020.10.011 PubMedGoogle ScholarCrossref
101.
Temkin  E; Healthcare Worker COVID-19 Surveillance Working Group.  Extremely low prevalence of asymptomatic COVID-19 among healthcare workers caring for COVID-19 patients in Israeli hospitals: a cross-sectional study.   Clin Microbiol Infect. 2021;27(1):130.e1-130.e4. doi:10.1016/j.cmi.2020.09.040PubMedGoogle ScholarCrossref
102.
Trahan  MJ, Mitric  C, Malhamé  I, Abenhaim  HA.  Screening and testing pregnant patients for SARS-CoV-2: first-wave experience of a designated COVID-19 hospitalization centre in Montreal.   J Obstet Gynaecol Canada. 2021;43(5):571-575. doi:10.1016/j.jogc.2020.11.001 PubMedGoogle Scholar
103.
Tsou  TP, Chen  WC, Huang  ASE, Chang  SC; Taiwan COVID-19 Outbreak Investigation Team.  Epidemiology of the first 100 cases of COVID-19 in Taiwan and its implications on outbreak control.   J Formos Med Assoc. 2020;119(11):1601-1607. doi:10.1016/j.jfma.2020.07.015 PubMedGoogle ScholarCrossref
104.
van Buul  LW, van den Besselaar  JH, Koene  FMHPH, Buurman  BM, Hertogh  CMPM; COVID-19 NH-Study Group; COVID-19 NH-Study Group.  Asymptomatic cases and limited transmission of SARS-CoV-2 in residents and healthcare workers in three Dutch nursing homes.   Gerontol Geriatr Med. 2020;6:2333721420982800. doi:10.1177/2333721420982800 PubMedGoogle Scholar
105.
Varnell  C  Jr, Harshman  LA, Smith  L,  et al.  COVID-19 in pediatric kidney transplantation: the Improving Renal Outcomes Collaborative.   Am J Transplant. 2021;21(8):2740-2748. doi:10.1111/ajt.16501 PubMedGoogle ScholarCrossref
106.
Wadhwa  A, Fisher  KA, Silver  R,  et al.  Identification of presymptomatic and asymptomatic cases using cohort-based testing approaches at a large correctional facility: Chicago, Illinois, USA, May 2020.   Clin Infect Dis. 2021;72(5):e128-e135. doi:10.1093/cid/ciaa1802PubMedGoogle ScholarCrossref
107.
Wi  YM, Lim  SJ, Kim  SH,  et al.  Response system for and epidemiological features of COVID-19 in Gyeongsangnam-do Province in South Korea.   Clin Infect Dis. 2021;72(4):661-667. PubMedGoogle ScholarCrossref
108.
Wood  J, Datta  D, Hudson  BL,  et al.  Prevalence of asymptomatic SARS-CoV-2 infection in children and adults in Marion County, Indiana.   Cureus. 2020;12(8):e9794. doi:10.7759/cureus.9794PubMedGoogle Scholar
109.
Yamahata  Y, Shibata  A.  Preparation for quarantine on the cruise ship Diamond Princess in Japan due to COVID-19.   JMIR Public Health Surveill. 2020;6(2):e18821. doi:10.2196/18821 PubMedGoogle Scholar
110.
Yassa  M, Yirmibes  C, Cavusoglu  G,  et al.  Outcomes of universal SARS-CoV-2 testing program in pregnant women admitted to hospital and the adjuvant role of lung ultrasound in screening: a prospective cohort study.   J Matern Fetal Neonatal Med. 2020;33(22):3820-3826. doi:10.1080/14767058.2020.1798398 PubMedGoogle ScholarCrossref
111.
Yau  K, Muller  MP, Lin  M,  et al.  COVID-19 outbreak in an urban hemodialysis unit.   Am J Kidney Dis. 2020;76(5):690-695.e1. doi:10.1053/j.ajkd.2020.07.001 PubMedGoogle ScholarCrossref
112.
Yousaf  AR, Duca  LM, Chu  V,  et al.  A prospective cohort study in nonhospitalized household contacts with severe acute respiratory syndrome coronavirus 2 infection: symptom profiles and symptom change over time.   Clin Infect Dis. 2021;73(7):e1841-e1849. doi:10.1093/cid/ciaa1072PubMedGoogle ScholarCrossref
113.
Zhang  T, Wu  HT, Wang  LH, Yang  WZ.  Scenario-based study of medical resource requirement rapid assessment under the COVID-19 pandemic  [in Chinese].  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(0):E059. doi:10.3760/cma.j.cn112338-20200401-00488PubMedGoogle Scholar
114.
Zhang  S, Guo  M, Wu  F,  et al.  Factors associated with asymptomatic infection in health-care workers with severe acute respiratory syndrome coronavirus 2 infection in Wuhan, China: a multicentre retrospective cohort study.   Clin Microbiol Infect. 2020;26(12):1670-1675. doi:10.1016/j.cmi.2020.08.038 PubMedGoogle ScholarCrossref
115.
Zhao  D, Wang  M, Wang  M,  et al.  Asymptomatic infection by SARS-CoV-2 in healthcare workers: a study in a large teaching hospital in Wuhan, China.   Int J Infect Dis. 2020;99:219-225. doi:10.1016/j.ijid.2020.07.082 PubMedGoogle ScholarCrossref
116.
Black  JRM, Bailey  C, Przewrocka  J, Dijkstra  KK, Swanton  C.  COVID-19: the case for health-care worker screening to prevent hospital transmission.   Lancet. 2020;395(10234):1418-1420. doi:10.1016/S0140-6736(20)30917-X PubMedGoogle ScholarCrossref
117.
Passarelli  VC, Faico-Filho  K, Moreira  LVL,  et al.  Asymptomatic COVID-19 in hospital visitors: the underestimated potential of viral shedding.   Int J Infect Dis. 2021;102:412-414. doi:10.1016/j.ijid.2020.10.057 PubMedGoogle ScholarCrossref
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    Original Investigation
    Infectious Diseases
    December 14, 2021

    Global Percentage of Asymptomatic SARS-CoV-2 Infections Among the Tested Population and Individuals With Confirmed COVID-19 Diagnosis: A Systematic Review and Meta-analysis

    Author Affiliations
    • 1Department of Epidemiology and Biostatistics, School of Public Health, Peking University, Beijing, China
    • 2School of Health Humanities, Peking University, Beijing, China
    JAMA Netw Open. 2021;4(12):e2137257. doi:10.1001/jamanetworkopen.2021.37257
    Key Points

    Question  What is the percentage of asymptomatic individuals with positive test results for SARS-CoV-2 among tested individuals and those with confirmed COVID-19 diagnosis?

    Findings  In this systematic review and meta-analysis of 95 unique studies with 29 776 306 individuals undergoing testing, the pooled percentage of asymptomatic infections was 0.25% among the tested population and 40.50% among the population with confirmed COVID-19.

    Meaning  The high percentage of asymptomatic infections from this study highlights the potential transmission risk of asymptomatic infections in communities.

    Abstract

    Importance  Asymptomatic infections are potential sources of transmission for COVID-19.

    Objective  To evaluate the percentage of asymptomatic infections among individuals undergoing testing (tested population) and those with confirmed COVID-19 (confirmed population).

    Data Sources  PubMed, EMBASE, and ScienceDirect were searched on February 4, 2021.

    Study Selection  Cross-sectional studies, cohort studies, case series studies, and case series on transmission reporting the number of asymptomatic infections among the tested and confirmed COVID-19 populations that were published in Chinese or English were included.

    Data Extraction and Synthesis  This meta-analysis was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. Random-effects models were used to estimate the pooled percentage and its 95% CI. Three researchers performed the data extraction independently.

    Main Outcomes and Measures  The percentage of asymptomatic infections among the tested and confirmed populations.

    Results  Ninety-five unique eligible studies were included, covering 29 776 306 individuals undergoing testing. The pooled percentage of asymptomatic infections among the tested population was 0.25% (95% CI, 0.23%-0.27%), which was higher in nursing home residents or staff (4.52% [95% CI, 4.15%-4.89%]), air or cruise travelers (2.02% [95% CI, 1.66%-2.38%]), and pregnant women (2.34% [95% CI, 1.89%-2.78%]). The pooled percentage of asymptomatic infections among the confirmed population was 40.50% (95% CI, 33.50%-47.50%), which was higher in pregnant women (54.11% [95% CI, 39.16%-69.05%]), air or cruise travelers (52.91% [95% CI, 36.08%-69.73%]), and nursing home residents or staff (47.53% [95% CI, 36.36%-58.70%]).

    Conclusions and Relevance  In this meta-analysis of the percentage of asymptomatic SARS-CoV-2 infections among populations tested for and with confirmed COVID-19, the pooled percentage of asymptomatic infections was 0.25% among the tested population and 40.50% among the confirmed population. The high percentage of asymptomatic infections highlights the potential transmission risk of asymptomatic infections in communities.

    Introduction

    COVID-19, the disease caused by SARS-CoV-2, was first reported in December 2019.1 Globally, as of January 28, 2021, there have been 100 455 529 confirmed cases, including 2 166 440 deaths.2 The disease course of COVID-19 ranges from asymptomatic to mild respiratory infections to pneumonia and even to acute respiratory distress syndrome.3 Patients with no symptoms at screening point were defined as having asymptomatic infections, which included infected people who have not yet developed symptoms but go on to develop symptoms later (presymptomatic infections), and those who are infected but never develop any symptoms (true asymptomatic or covert infections).4,5 Owing to the absence of symptoms, these patients would not seek medical care and could not be detected by temperature screening. Presymptomatic transmission will also make temperature screening less effective.6 Only extensive testing and close contact tracing could lead to identification of more asymptomatic infections.7

    Unlike SARS, which had little known transmission from asymptomatic patients, evidence showed that asymptomatic patients were a potential source of transmission of COVID-19.3,6 A previous study8 showed that the upper respiratory viral loads in asymptomatic patients were comparable to those in symptomatic patients. Meanwhile, the highest viral load in throat swabs at the time of symptom onset indicated that infectiousness peaked on or before symptom onset.9 Moreover, studies showed that asymptomatic infections might have contributed to transmission among households, nursing facilities, and clusters.10-13 As the pandemic has been contained in many countries and regions, travel restrictions have been lifted and public places have reopened. Asymptomatic infections should be considered a source of COVID-19 infections that play an important role in the spread of the virus within community as public life gradually returns to normal. The management of asymptomatic carriers was essential for preventing cluster outbreaks and transmission within a community.

    However, comprehensive evaluation of the percentage of asymptomatic infections among the tested population and the population with confirmed COVID-19 (confirmed population) is limited. Current results from different studies3,5,7,8,10,11 varied considerably owing to different study design and study population. Thus, we conducted a meta-analysis to better understand the global percentage of asymptomatic infections among the tested and confirmed COVID-19 populations. Our results could be useful for strategies to reduce transmission by asymptomatic infections.

    Methods
    Search Strategy

    We conducted the meta-analysis following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. This review was not registered. Three researchers (Q.L., L.K., and R.L.) searched the published studies on February 4, 2021, through PubMed, EMBASE, and ScienceDirect without language restriction. The search terms used included COVID-19, coronavirus, SARS-CoV-2, asymptomatic transmission, asymptomatic infection, asymptomatic proportion, asymptomatic case, asymptomatic cases, asymptomatic contact, asymptomatic ratio, asymptomatic people, asymptomatic patients, and asymptomatic patient. The detailed search strategies are shown in eMethods 1 in the Supplement. Three researchers (Q.L., L.K., and R.L.) reviewed the titles, abstracts, and full texts of articles independently and identified additional studies from the reference lists. Disagreements were resolved by 2 other reviewers (W.J. and Y.W.).

    Selection Criteria

    Asymptomatic individuals with positive test results for SARS-CoV-2 (asymptomatic infections) were defined as those who did not present any symptoms at the time of SARS-CoV-2 testing or diagnosis.14 Individuals with a confirmed COVID-19 diagnosis were defined as those who had a throat swab or other specimen with positive results for SARS-CoV-2 using a real-time reverse-transcription polymerase chain reaction assay. Inclusion criteria consisted of (1) studies reporting the number of asymptomatic infections, tested population, and confirmed population and (2) cross-sectional studies, cohort studies, case series studies, and case series on transmission. Exclusion criteria consisted of (1) reviews, systematic reviews, and meta-analysis; (2) duplicate publications; (3) preprints; (4) multiple studies reporting on overlapping participants (the study with more information was included); (5) articles with ambiguous definition of asymptomatic infections; and (6) articles not written in English or Chinese.

    Data Extraction and Quality Assessment

    Three researchers (Q.L., L.K., and R.L.) performed the data extraction independently. Data were extracted for the first author, date of publication, study location, number of tested individuals, number of individuals with confirmed COVID-19, and number of asymptomatic infections. The ratio of male to female individuals (MFR) and mean age of study participants were gathered if available. The quality of studies included in the meta-analysis was assessed using the Joanna Briggs Institute Prevalence Critical Appraisal Tool15 for cross-sectional studies and the Newcastle-Ottawa scale16 for cohort studies (eMethods 2 in the Supplement). Case series on transmission were assessed using the quality assessment tool developed by Yanes-Lane et al.17 Two researchers (Q.L. and L.K.) performed the quality assessment independently. Disagreements were resolved by 2 other reviewers (W.J. and Y.W.). Outcomes of interest included the percentages of asymptomatic infections among the tested and the confirmed populations.

    Statistical Analysis

    We performed a meta-analysis to estimate the pooled percentage of asymptomatic infections among the tested and confirmed populations. Untransformed percentages and DerSimonian and Laird random-effects models18 were used to calculate the pooled percentage and its 95% CI. The heterogeneity among studies was assessed using I2 values.19 We performed subgroup analyses by study location (Africa, Asia, Europe, North America, and South America), countries’ development level (developed vs developing), study population (air or cruise travelers, close contact, community residents, health care workers or in-hospital patients, nursing home residents or staff, and pregnant women), publication period (June 2020 and earlier vs July 2020 and later), sample size for the tested population (1-99, 100-999, 1000-9999, and ≥10 000), sample size for the confirmed population (1-99, 100-499, and ≥500), study design (case series, case series on transmission, cohort studies, and cross-sectional studies), study quality (low, moderate, and high), MFR (0 to <0.5, 0.5 to <1.0, 1.0 to <1.5, and ≥1.5), and mean age (<20, 20-39, 40-59, and ≥60 years). Publication bias was assessed by funnel plot and the Egger regression test.20 We performed 3 sensitivity analyses to test the robustness of our results, by using the Knapp-Hartung adjustments21 to calculate the 95% CIs around the pooled effects, by excluding 3 studies with a tested population more than 200 000 and studies with low quality. Two-sided P < .05 indicated statistical significance. All analyses were performed using R, version 4.0.0 (R Project for Statistical Computing).

    Results

    We identified 2860 studies through database search and the reference lists of articles and reviews. Of these, 282 studies underwent full-text review. Ninety-five studies with information concerning the percentage of asymptomatic infections among the tested and confirmed populations were included in the final analysis12,22-115 (Figure 1).

    Among these studies, 44 (46.32%) were cross-sectional studies, 41 (43.16%) were cohort studies, 7 (7.37%) were case series, and 3 (3.16%) were case series on transmission studies. Thirty-five studies (36.84%) were conducted in Europe; 32 (33.68%), in North America; and 25 (26.32%), in Asia. Seventy-four studies (77.89%) were conducted in developed countries. Thirty-seven studies (38.95%) were conducted among health care workers or in-hospital patients; 17 (17.89%), among nursing home residents or staff; 14 (14.74%), among community residents; 13 (13.68%), among pregnant women; 8 (8.42%), among air or cruise travelers; and 6 (6.32%), among close contacts. Twenty-one studies (22.11%) were published in June or before; 74 (77.89%), in July and after. Forty-nine studies (51.58%) had sample size of 100 to 1000. Fifty-three studies (55.79%) were assessed as low quality; 17 (17.89%), high quality; and 25 (26.32%), moderate quality (Table). For cross-sectional studies, low-quality studies were mostly those without random sampling or with 2 or more biases (selection bias, reporting bias, or detection bias). For cohort studies, low-quality studies were mostly those with 1 or more biases.

    Percentage of Asymptomatic Infections Among the Tested Population

    Ninety-five studies were included in the meta-analysis for the percentage of asymptomatic infections among the tested population, covering 29 776 306 tested individuals, among whom 11 516 had asymptomatic infections. The pooled percentage of asymptomatic infections among the tested population was 0.25% (95% CI, 0.23%-0.27%), with high heterogeneity among studies (I2 = 99%; P < .001) (eFigure 1 in the Supplement).

    Among tested individuals in different study populations, the pooled percentage of asymptomatic infections was 4.52% (95% CI, 4.15%-4.89%) in nursing home residents or staff, 2.02% (95% CI, 1.66%-2.38%) in air or cruise travelers, 2.34% (95% CI, 1.89%-2.78%) in pregnant women, 1.46% (95% CI, 1.05%-1.88%) in close contacts, 0.75% (95% CI, 0.60%-0.90%) in health care workers or in-hospital patients, and 0.40% (95% CI, 0.18%-0.62%) in community residents. The pooled percentage of asymptomatic infections was 0.90% (95% CI, 0.87%-0.93%) in Europe, 0.47% (95% CI, 0.39%-0.54%) in North America, and 0.05% (95% CI, 0.04%-0.07%) in Asia. The pooled percentage was higher in developed countries (0.70% [95% CI, 0.67%-0.73%]), studies published in July or later (0.29% [95% CI, 0.27%-0.31%]), studies with a sample size of less than 100 (6.74% [95% CI, 4.69%-8.80%]), and cohort studies (2.98% [95% CI, 2.68%-3.29%]). In studies with MFR of 0.5 to less than 1.0, the pooled percentage was higher (3.91%; [95% CI, 3.14%-4.68%]). The pooled percentage was higher when the mean age of the study population was 60 years or older (3.69% [95% CI, 2.99%-4.39%]) (Figure 2).

    Percentage of Asymptomatic Infections Among the Confirmed Population

    Among 95 studies, 18 were excluded because that the percentage of asymptomatic infections among the confirmed population was 100%.22,30,31,36,39,43,49,56,58,71,77,79-81,87,96,101,108 The remaining 77 studies were included in the meta-analysis for the percentage of asymptomatic infections among the confirmed population,12,23-29,32-35,37,38,40-42,44-48,50-55,57,59-70,72-76,78,82-86,88-95,97-100,102-107,109-115 covering 19 884 individuals with confirmed COVID-19, among whom 11 069 had asymptomatic infections. The pooled percentage of asymptomatic infections among the confirmed population was 40.50% (95% CI, 33.50%-47.50%), with high heterogeneity among studies (I2 = 99%; P < .001) (eFigure 2 in the Supplement).

    Among the confirmed population, the pooled percentage of asymptomatic infections was 54.11% (95% CI, 39.16%-69.05%) in pregnant women, 52.91% (95% CI, 36.08%-69.73%) in air or cruise travelers, 47.53% (95% CI, 36.36%-58.70%) in nursing home residents or staff, 39.74% (95% CI, 24.50%-54.98%) in community residents, 30.01% (95% CI, 21.13%-38.88%) in health care workers or in-hospital patients, and 26.94% (95% CI, 8.50%-45.38%) in close contacts. The pooled percentage of asymptomatic infections was 46.32% (95% CI, 33.47%-59.16%) in North America, 44.18% (95% CI, 32.87%-55.50%) in Europe, and 27.58% (95% CI, 13.60%-41.57%) in Asia. The pooled percentage was higher in developed countries (43.51% [95% CI, 35.59%-51.44%]), studies published in June or earlier (43.68% [95% CI, 27.87%-59.50%]), studies with sample size of 500 or greater (47.06% [95% CI, 26.22%-67.90%]), and cross-sectional studies (44.47% [95% CI, 33.54%-55.40%]). The pooled percentage was slightly lower for cohort studies (40.96% [95% CI, 31.18%-50.74%]). Among studies with MFR of 1.0 to less than 1.5, the pooled percentage was higher (55.09% [95% CI, 27.64%-82.53%]). The pooled percentage was higher when the mean age of the study population was younger than 20 years (60.21% [95% CI, 24.51%-95.91%]) or 20 to 39 years (49.49% [95% CI, 33.48%-65.50%]) (Figure 3).

    Sensitivity Analysis and Publication Bias

    After using the Knapp-Hartung adjustments, the pooled percentage of asymptomatic infections among the tested population was 0.25% (95% CI, 0.11%-0.39%), and the 95% CI of the pooled percentage became slightly larger (eFigure 3 in the Supplement). The percentage of asymptomatic infections among the confirmed population was 40.50% (95% CI, 34.94%-46.07%), and the 95% CI of the pooled percentage became slightly narrower (eFigure 4 in the Supplement).

    After excluding 3 studies with tested populations of more than 200 000,36,52,90 the pooled percentage of asymptomatic infections among the tested population was 1.61% (95% CI, 1.47%-1.76%), which was higher than the original results. The percentage of asymptomatic infections among the confirmed population was 39.37% (95% CI, 33.86%-44.87%), which was slightly lower than the original results. After excluding 53 low-quality studies, the pooled percentage of asymptomatic infections among the tested population was 0.24% (95% CI, 0.23%-0.26%), and the percentage of asymptomatic infections among the confirmed population was 41.71% (95% CI, 31.89%-51.53%). Both percentages were similar to the original results.

    Funnel plots are shown in Figure 4. Egger regression tests for the percentage of asymptomatic infections among the tested population (z = 43.1725; P < .001) and for the percentage of asymptomatic infections among the confirmed population (z = 2.3846; P = .02) indicated that there might be publication bias.

    Discussion

    In this meta-analysis, we found that the pooled percentage of asymptomatic infections among the tested population was 0.25% (95% CI, 0.23%-0.27%), and the pooled percentage of asymptomatic infections among the confirmed population was 40.50% (95% CI, 33.50%-47.50%). At present, there are only a few meta-analyses for the percentage of asymptomatic infections among the tested population. We found that the percentage of asymptomatic infections was highest among the tested population in nursing homes and lowest among community residents. Because the percentage of asymptomatic individuals varies as a function of community prevalence, it was not available in all studies. This might be a potential driver of heterogeneity across studies. Furthermore, the percentages of asymptomatic infections among the tested population were different between studies conducted in different locations. Studies in Asia had the lowest percentage, whereas studies in other locations had higher percentages. This lower percentage in Asia might be related to the large city-wide SARS-CoV-2 nucleic acid screening program in China.36 In the sensitivity analyses, we found that the pooled percentage of asymptomatic infections among the tested population was higher than the original results after excluding studies with large sample sizes. This indicated that studies with different sample sizes were very heterogeneous. Owing to severe outcomes among older patients with COVID-19, more studies were conducted among nursing home residents or staff. Thus, asymptomatic individuals were more likely to be tested among this population. As more and more countries conducted expanded screening, studies concerning the percentage of asymptomatic infections among the general population would increase in the future.

    In this study, the pooled percentage of asymptomatic infections among the confirmed population was 40.50%. The pooled percentage of asymptomatic infections was 40.96% among cohort studies, which was slightly lower than that among cross-sectional studies (44.47%). The patients who developed symptoms later were mistakenly classified as having asymptomatic infection in cross-sectional studies because the observation time was not long enough.14 Thus, the percentage of asymptomatic infections was lower in cohort studies, because some patients with presymptomatic findings were identified during follow-up. There were limited case series of great interest in the first months of the pandemic; however, these studies mostly traced and tested limited contacts, which contributed limited value to the evidence of the percentage of asymptomatic infections.17 Several meta-analyses concerned the percentage of asymptomatic infections among the confirmed population. Chen et al5 conducted a meta-analysis that included 104 published studies and preprints before May 13, 2020. They found that the percentage of asymptomatic individuals among those with COVID-19 was 13.34% (95% CI, 10.86%-16.29%). Unlike our study, Chen et al5 searched a Chinese database. Thus, the percentage of Chinese studies was higher in their study than in the present study. He et al14 searched PubMed and Embase before May 20, 2020, and included 41 published studies. More than 50% of the studies were from China, and the pooled percentage of asymptomatic infection was 15.6% (95% CI, 10.1%-23.0%). In our study, we only included published studies. The percentage of countries excluding China was higher than the previous meta-analysis.14 This might be the reason for the higher percentage of asymptomatic infections found in our study compared with studies conducted by Chen et al5 and He et al.14 Another meta-analysis conducted by Yanes-Lane et al17 included published studies and preprints before June 22, 2020. After quality assessment, 28 studies were of high or moderate quality and were included in the meta-analysis. The percentage of asymptomatic infection among persons with confirmed COVID-19 varied among different study populations, with the highest observed in obstetric patients (95% [95% CI, 45%-100%]).

    In our study, the percentage of asymptomatic infections among the confirmed population was 54.11% in pregnant women and 52.91% in air or cruise travelers. The percentage was 47.53% in nursing home residents or staff. This finding of a high percentage of asymptomatic infections among air or cruise travelers suggests that screening and quarantine on airport arrival is important for reducing community transmissions, especially in countries without local transmission.3,25 In addition, we found that the percentage of asymptomatic infections among the tested population was relatively low among community residents. However, the percentage of asymptomatic infection among confirmed individuals was 39.74% in communities. These findings suggest that asymptomatic infections might contribute to the transmission of SARS-CoV-2 within the community. To prevent further transmission in communities, asymptomatic individuals among the general population should be tested. If resources are limited, workers in specific industries such as air transportation should be routinely tested. In addition, we found that approximately one-third of individuals with confirmed COVID-19 were asymptomatic among health care workers or in-hospital patients. Because asymptomatic health care workers might contribute to disease spread in and out of hospitals, surveillance of asymptomatic individuals is important for infection control and transmission reduction in health care settings and community.116,117 Meanwhile, hand hygiene and personal protective equipment were necessary for hospital visitors.117 A previous study showed that most asymptomatic patients belong to younger groups,3 which was consistent with the findings of our study. The percentage of asymptomatic infections was higher among groups younger than 39 years than in other age groups, possibly because the young adults were more likely to show only mild or moderate clinical symptoms.5 This indicated that young adults who often presented mild or no symptoms were a potential source of transmission in the community.

    In the meta-analysis, we included studies published before February 3, 2021, providing the most updated pooled percentage of asymptomatic infections among tested and confirmed populations. We included countries in Africa, Asia, Europe, North America, and South America and estimated the percentage of asymptomatic infections for different populations. Our results could raise awareness among the public and policy makers and provide evidence for prevention strategies.

    Limitations

    This study has several limitations. First, we did not include preprints and therefore may have missed some relevant studies; however, we thought that the results of published studies were more reliable. Second, some relevant articles written in Chinese may not be included because we did not search Chinese literature databases such as China National Knowledge Infrastructure. Third, most studies did not follow up to identify presymptomatic and covert infections. Future studies should evaluate the percentage of these 2 types of asymptomatic infection among the confirmed population. Fourth, most studies were conducted in a specific population; thus, our findings might not be generalizable to the general population. Fifth, the heterogeneity between studies was high, which might be related to different study location, period, population, and sample size. Sixth, the Egger regression test suggested potential publication bias in this study. Because studies that did not detect asymptomatic infections were less likely to be published, our pooled percentage of asymptomatic infections might be overestimated.

    Conclusions

    In this systematic review and meta-analysis, we found that the pooled percentage of asymptomatic SARS-CoV-2 infections among the tested population was 0.25%. Among the confirmed population, 40.50% of individuals had asymptomatic infections. The high percentage of asymptomatic infections highlights the potential transmission risk of asymptomatic infections in communities. Screening for asymptomatic infection is required, especially for countries and regions that have successfully controlled SARS-CoV-2. Asymptomatic infections should be under management similar to that for confirmed infections, including isolating and contact tracing.

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    Article Information

    Accepted for Publication: October 8, 2021.

    Published: December 14, 2021. doi:10.1001/jamanetworkopen.2021.37257

    Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2021 Ma Q et al. JAMA Network Open.

    Corresponding Author: Min Liu, PhD, Department of Epidemiology and Biostatistics, School of Public Health, Peking University, 38 Xueyuan Rd, Haidian District, Beijing 100191, China (liumin@bjmu.edu.cn).

    Author Contributions: Dr M. Liu had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Drs Ma and J. Liu contributed equally to this study and are co–first authors.

    Concept and design: Ma, J. Liu, Jing, M. Liu.

    Acquisition, analysis, or interpretation of data: Ma, Q. Liu, Kang, R. Liu, Wu, M. Liu.

    Drafting of the manuscript: Ma, Q. Liu, Kang, R. Liu.

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

    Statistical analysis: Ma, J. Liu, Q. Liu, Jing.

    Obtained funding: J. Liu, M. Liu.

    Supervision: M. Liu.

    Conflict of Interest Disclosures: None reported.

    Funding/Support: This work was supported by grants 71934002, 71874003, and 72122001 from the National Natural Science Foundation of China.

    Role of the Funder/Sponsor: The sponsor 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.

    References
    1.
    Huang  C, Wang  Y, Li  X,  et al.  Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China.   Lancet. 2020;395(10223):497-506. doi:10.1016/S0140-6736(20)30183-5 PubMedGoogle ScholarCrossref
    2.
    World Health Organization. WHO coronavirus disease (COVID-19) dashboard. Accessed January 28, 2021. https://covid19.who.int/
    3.
    Kronbichler  A, Kresse  D, Yoon  S, Lee  KH, Effenberger  M, Shin  JI.  Asymptomatic patients as a source of COVID-19 infections: a systematic review and meta-analysis.   Int J Infect Dis. 2020;98:180-186. doi:10.1016/j.ijid.2020.06.052 PubMedGoogle ScholarCrossref
    4.
    World Health Organization. Coronavirus disease (COVID-19): how is it transmitted? December 13, 2020. Accessed January 22, 2021. https://www.who.int/news-room/q-a-detail/coronavirus-disease-covid-19-how-is-it-transmitted
    5.
    Chen  C, Zhu  C, Yan  D,  et al.  The epidemiological and radiographical characteristics of asymptomatic infections with the novel coronavirus (COVID-19): a systematic review and meta-analysis.   Int J Infect Dis. 2021;104:458-464. doi:10.1016/j.ijid.2021.01.017 PubMedGoogle ScholarCrossref
    6.
    Wilder-Smith  A, Chiew  CJ, Lee  VJ.  Can we contain the COVID-19 outbreak with the same measures as for SARS?   Lancet Infect Dis. 2020;20(5):e102-e107. doi:10.1016/S1473-3099(20)30129-8 PubMedGoogle ScholarCrossref
    7.
    Krishnasamy  N, Natarajan  M, Ramachandran  A,  et al.  Clinical outcomes among asymptomatic or mildly symptomatic COVID-19 patients in an isolation facility in Chennai, India.   Am J Trop Med Hyg. 2021;104(1):85-90. doi:10.4269/ajtmh.20-1096 PubMedGoogle ScholarCrossref
    8.
    Ra  SH, Lim  JS, Kim  GU, Kim  MJ, Jung  J, Kim  SH.  Upper respiratory viral load in asymptomatic individuals and mildly symptomatic patients with SARS-CoV-2 infection.   Thorax. 2021;76(1):61-63. doi:10.1136/thoraxjnl-2020-215042 PubMedGoogle ScholarCrossref
    9.
    He  X, Lau  EHY, Wu  P,  et al.  Temporal dynamics in viral shedding and transmissibility of COVID-19.   Nat Med. 2020;26(5):672-675. doi:10.1038/s41591-020-0869-5 PubMedGoogle ScholarCrossref
    10.
    Bai  Y, Yao  L, Wei  T,  et al.  Presumed asymptomatic carrier transmission of COVID-19.   JAMA. 2020;323(14):1406-1407. doi:10.1001/jama.2020.2565 PubMedGoogle ScholarCrossref
    11.
    Tong  ZD, Tang  A, Li  KF,  et al.  Potential presymptomatic transmission of SARS-CoV-2, Zhejiang Province, China, 2020.   Emerg Infect Dis. 2020;26(5):1052-1054. doi:10.3201/eid2605.200198 PubMedGoogle ScholarCrossref
    12.
    Arons  MM, Hatfield  KM, Reddy  SC,  et al; Public Health–Seattle and King County and CDC COVID-19 Investigation Team.  Presymptomatic SARS-CoV-2 infections and transmission in a skilled nursing facility.   N Engl J Med. 2020;382(22):2081-2090. doi:10.1056/NEJMoa2008457 PubMedGoogle ScholarCrossref
    13.
    Zhang  Y, Muscatello  D, Tian  Y,  et al.  Role of presymptomatic transmission of COVID-19: evidence from Beijing, China.   J Epidemiol Community Health. 2021;75(1):84-87. doi:10.1136/jech-2020-214635PubMedGoogle Scholar
    14.
    He  J, Guo  Y, Mao  R, Zhang  J.  Proportion of asymptomatic coronavirus disease 2019: a systematic review and meta-analysis.   J Med Virol. 2021;93(2):820-830. doi:10.1002/jmv.26326 PubMedGoogle ScholarCrossref
    15.
    Moola  S, Munn  Z, Tufanaru  C,  et al. Chapter 7: Systematic reviews of etiology and risk. In: Aromataris E, Munn Z, eds. JBI Manual for Evidence Synthesis. JBI; 2020. Accessed September 5, 2021. https://jbi-global-wiki.refined.site/space/MANUAL
    16.
    Wells  GA, Shea  B, O'Connell  D, Peterson  J, Welch  V, Losos  M, Tugwell  P.  The Newcastle-Ottawa Scale (NOS) for Assessing the Quality of Nonrandomised Studies in Meta-analyses. Ottawa Hospital Research Institute; 2011. Accessed September 5, 2021. http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp
    17.
    Yanes-Lane  M, Winters  N, Fregonese  F,  et al.  Proportion of asymptomatic infection among COVID-19 positive persons and their transmission potential: a systematic review and meta-analysis.   PLoS One. 2020;15(11):e0241536. doi:10.1371/journal.pone.0241536 PubMedGoogle Scholar
    18.
    DerSimonian  R, Laird  N.  Meta-analysis in clinical trials revisited.   Contemp Clin Trials. 2015;45(Pt A):139-145. doi:10.1016/j.cct.2015.09.002PubMedGoogle Scholar
    19.
    Higgins  JP, Thompson  SG, Deeks  JJ, Altman  DG.  Measuring inconsistency in meta-analyses.   BMJ. 2003;327(7414):557-560. doi:10.1136/bmj.327.7414.557 PubMedGoogle ScholarCrossref
    20.
    Egger  M, Davey Smith  G, Schneider  M, Minder  C.  Bias in meta-analysis detected by a simple, graphical test.   BMJ. 1997;315(7109):629-634. doi:10.1136/bmj.315.7109.629 PubMedGoogle ScholarCrossref
    21.
    Knapp  G, Hartung  J.  Improved tests for a random effects meta-regression with a single covariate.   Stat Med. 2003;22(17):2693-2710. doi:10.1002/sim.1482 PubMedGoogle ScholarCrossref
    22.
    Abdelmoniem  R, Fouad  R, Shawky  S,  et al.  SARS-CoV-2 infection among asymptomatic healthcare workers of the emergency department in a tertiary care facility.   J Clin Virol. 2021;134:104710. doi:10.1016/j.jcv.2020.104710 PubMedGoogle Scholar
    23.
    Abeysuriya  S, Wasif  S, Counihan  C,  et al.  Universal screening for SARS-CoV-2 in pregnant women at term admitted to an East London maternity unit.   Eur J Obstet Gynecol Reprod Biol. 2020;252:444-446. doi:10.1016/j.ejogrb.2020.07.035 PubMedGoogle ScholarCrossref
    24.
    Akbarialiabad  H, Abdolrahimzadeh Fard  H, Abbasi  HR,  et al.  Our experience of trauma management during novel coronovirus 2019 (COVID-19) pandemic in a busy trauma center in southern Iran.   Bull Emerg Trauma. 2020;8(3):199-201.PubMedGoogle Scholar
    25.
    Al-Qahtani  M, AlAli  S, AbdulRahman  A, Salman Alsayyad  A, Otoom  S, Atkin  SL.  The prevalence of asymptomatic and symptomatic COVID-19 in a cohort of quarantined subjects.   Int J Infect Dis. 2021;102:285-288. doi:10.1016/j.ijid.2020.10.091 PubMedGoogle ScholarCrossref
    26.
    Al-Shamsi  HO, Coomes  EA, Aldhaheri  K, Alrawi  S.  Serial screening for COVID-19 in asymptomatic patients receiving anticancer therapy in the United Arab Emirates.   JAMA Oncol. 2021;7(1):129-131. doi:10.1001/jamaoncol.2020.5745 PubMedGoogle ScholarCrossref
    27.
    Arnold  FW, Bishop  S, Oppy  L, Scott  L, Stevenson  G.  Surveillance testing reveals a significant proportion of hospitalized patients with SARS-CoV-2 are asymptomatic.   Am J Infect Control. 2021;49(3):281-285. doi:10.1016/j.ajic.2021.01.005 PubMedGoogle ScholarCrossref
    28.
    Aslam  A, Singh  J, Robilotti  E,  et al.  SARS CoV-2 surveillance and exposure in the perioperative setting with universal testing and personal protective equipment (PPE) policies.   Clin Infect Dis. Published online October 22, 2020. doi:10.1093/cid/ciaa1607 Google Scholar
    29.
    Bayle  C, Cantin  D, Vidal  JS,  et al; APHP COVID 19 research collaboration.  Asymptomatic SARS COV-2 carriers among nursing home staff: A source of contamination for residents?   Infect Dis Now. 2021;51(2):197-200. doi:10.1016/j.idnow.2020.11.008PubMedGoogle ScholarCrossref
    30.
    Bender  WR, Hirshberg  A, Coutifaris  P, Acker  AL, Srinivas  SK.  Universal testing for severe acute respiratory syndrome coronavirus 2 in 2 Philadelphia hospitals: carrier prevalence and symptom development over 2 weeks.   Am J Obstet Gynecol MFM. 2020;2(4):100226. doi:10.1016/j.ajogmf.2020.100226 PubMedGoogle Scholar
    31.
    Bianco  A, Buckley  AB, Overbey  J,  et al.  Testing of patients and support persons for coronavirus disease 2019 (COVID-19) infection before scheduled deliveries.   Obstet Gynecol. 2020;136(2):283-287. doi:10.1097/AOG.0000000000003985 PubMedGoogle ScholarCrossref
    32.
    Blain  H, Rolland  Y, Tuaillon  E,  et al.  Efficacy of a test-retest strategy in residents and health care personnel of a nursing home facing a COVID-19 outbreak.   J Am Med Dir Assoc. 2020;21(7):933-936. doi:10.1016/j.jamda.2020.06.013 PubMedGoogle ScholarCrossref
    33.
    Blitz  MJ, Rochelson  B, Rausch  AC,  et al.  Universal testing for coronavirus disease 2019 in pregnant women admitted for delivery: prevalence of peripartum infection and rate of asymptomatic carriers at four New York hospitals within an integrated healthcare system.   Am J Obstet Gynecol MFM. 2020;2(3):100169. doi:10.1016/j.ajogmf.2020.100169 PubMedGoogle Scholar
    34.
    Blumberg  TJ, Adler  AC, Lin  EE,  et al.  Universal screening for COVID-19 in children undergoing orthopaedic surgery: a multicenter report.   J Pediatr Orthop. 2020;40(10):e990-e993. doi:10.1097/BPO.0000000000001657 PubMedGoogle ScholarCrossref
    35.
    Bosworth  A, Whalley  C, Poxon  C,  et al.  Rapid implementation and validation of a cold-chain free SARS-CoV-2 diagnostic testing workflow to support surge capacity.   J Clin Virol. 2020;128:104469. doi:10.1016/j.jcv.2020.104469 PubMedGoogle Scholar
    36.
    Cao  S, Gan  Y, Wang  C,  et al.  Post-lockdown SARS-CoV-2 nucleic acid screening in nearly ten million residents of Wuhan, China.   Nat Commun. 2020;11(1):5917. doi:10.1038/s41467-020-19802-w PubMedGoogle ScholarCrossref
    37.
    Carroll  C, Conway  R, O’Donnell  D,  et al.  Routine testing of close contacts of confirmed COVID-19 cases: national COVID-19 contact management programme, Ireland, May to August 2020.   Public Health. 2021;190:147-151. PubMedGoogle ScholarCrossref
    38.
    Cattelan  AM, Sasset  L, Di Meco  E,  et al.  An integrated strategy for the prevention of SARS-CoV-2 infection in healthcare workers: a prospective observational study.   Int J Environ Res Public Health. 2020;17(16):E5785. doi:10.3390/ijerph17165785 PubMedGoogle Scholar
    39.
    Cloutier  L, Merindol  N, Pépin  G,  et al.  Asymptomatic carriers of COVID-19 in a confined adult community population in Quebec: a cross-sectional study.   Am J Infect Control. 2021;49(1):120-122. doi:10.1016/j.ajic.2020.08.015 PubMedGoogle ScholarCrossref
    40.
    Corcorran  MA, Olin  S, Rani  G,  et al.  Prolonged persistence of PCR-detectable virus during an outbreak of SARS-CoV-2 in an inpatient geriatric psychiatry unit in King County, Washington.   Am J Infect Control. 2021;49(3):293-298. PubMedGoogle ScholarCrossref
    41.
    Deng  ZQ, Xia  W, Fan  YB,  et al.  Analysis on transmission chain of a cluster epidemic of COVID-19, Nanchang [in Chinese].   Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(9):1420-1423.PubMedGoogle Scholar
    42.
    Dora  AV, Winnett  A, Jatt  LP,  et al.  Universal and serial laboratory testing for SARS-CoV-2 at a long-term care skilled nursing facility for veterans: Los Angeles, California, 2020.   MMWR Morb Mortal Wkly Rep. 2020;69(21):651-655. doi:10.15585/mmwr.mm6921e1 PubMedGoogle ScholarCrossref
    43.
    Duan  P, Deng  ZQ, Pan  ZY, Wang  YP.  Safety considerations during return to work in the context of stable COVID-19 epidemic control: an analysis of health screening results of all returned staff from a hospital.   Epidemiol Infect. 2020;148:e214. doi:10.1017/S0950268820002150 PubMedGoogle Scholar
    44.
    Figueiredo  R, Tavares  S, Moucho  M, Ramalho  C.  Systematic screening for SARS-CoV-2 in pregnant women admitted for delivery in a Portuguese maternity.   J Perinat Med. 2020;48(9):977-980. doi:10.1515/jpm-2020-0387 PubMedGoogle ScholarCrossref
    45.
    Goldfarb  IT, Diouf  K, Barth  WH,  et al.  Universal SARS-CoV-2 testing on admission to the labor and delivery unit: low prevalence among asymptomatic obstetric patients.   Infect Control Hosp Epidemiol. 2020;41(9):1095-1096. doi:10.1017/ice.2020.255 PubMedGoogle ScholarCrossref
    46.
    Graham  NSN, Junghans  C, Downes  R,  et al.  SARS-CoV-2 infection, clinical features and outcome of COVID-19 in United Kingdom nursing homes.   J Infect. 2020;81(3):411-419. doi:10.1016/j.jinf.2020.05.073 PubMedGoogle ScholarCrossref
    47.
    Grechukhina  O, Greenberg  V, Lundsberg  LS,  et al.  Coronavirus disease 2019 pregnancy outcomes in a racially and ethnically diverse population.   Am J Obstet Gynecol MFM. 2020;2(4)(suppl):100246. doi:10.1016/j.ajogmf.2020.100246 PubMedGoogle Scholar
    48.
    Gruskay  JA, Dvorzhinskiy  A, Konnaris  MA,  et al.  Universal testing for COVID-19 in essential orthopaedic surgery reveals a high percentage of asymptomatic infections.   J Bone Joint Surg Am. 2020;102(16):1379-1388. doi:10.2106/JBJS.20.01053 PubMedGoogle ScholarCrossref
    49.
    Han  X, Wei  X, Alwalid  O,  et al.  Severe acute respiratory syndrome coronavirus 2 among asymptomatic workers screened for work resumption, China.   Emerg Infect Dis. 2020;26(9):2265-2267. doi:10.3201/eid2609.201848 PubMedGoogle ScholarCrossref
    50.
    Harada  S, Uno  S, Ando  T,  et al.  Control of a nosocomial outbreak of COVID-19 in a university hospital.   Open Forum Infect Dis. 2020;7(12):a512. doi:10.1093/ofid/ofaa512PubMedGoogle ScholarCrossref
    51.
    Hcini  N, Maamri  F, Picone  O,  et al.  Maternal, fetal and neonatal outcomes of large series of SARS-CoV-2 positive pregnancies in peripartum period: a single-center prospective comparative study.   Eur J Obstet Gynecol Reprod Biol. 2021;257:11-18. doi:10.1016/j.ejogrb.2020.11.068 PubMedGoogle ScholarCrossref
    52.
    Hoxha  A, Wyndham-Thomas  C, Klamer  S,  et al.  Asymptomatic SARS-CoV-2 infection in Belgian long-term care facilities.   Lancet Infect Dis. 2021;21(4):e67. doi:10.1016/S1473-3099(20)30560-0PubMedGoogle Scholar
    53.
    Hung  IFN, Cheng  VCC, Li  X,  et al.  SARS-CoV-2 shedding and seroconversion among passengers quarantined after disembarking a cruise ship: a case series.   Lancet Infect Dis. 2020;20(9):1051-1060. doi:10.1016/S1473-3099(20)30364-9 PubMedGoogle ScholarCrossref
    54.
    Ibrahim  F, Natasha  A, Saharman  YR, Sudarmono  P.  Preliminary report of COVID-19 testing: experience of the clinical microbiology laboratory Universitas Indonesia, Jakarta, Indonesia.   New Microbes New Infect. 2020;37:100733. doi:10.1016/j.nmni.2020.100733 PubMedGoogle Scholar
    55.
    Kennelly  SP, Dyer  AH, Noonan  C,  et al.  Asymptomatic carriage rates and case-fatality of SARS-CoV-2 infection in residents and staff in Irish nursing homes.   Age Ageing. 2021;50(1):49-54. doi:10.1093/ageing/afaa220PubMedGoogle ScholarCrossref
    56.
    Kessler  T, Wiebe  J, Graf  T, Schunkert  H, Kastrati  A, Sager  HB.  SARS-CoV-2 infection in asymptomatic patients hospitalized for cardiac emergencies: implications for patient management.   Front Cardiovasc Med. 2020;7:599299. doi:10.3389/fcvm.2020.599299 PubMedGoogle Scholar
    57.
    Kimball  A, Hatfield  KM, Arons  M,  et al; Public Health–Seattle & King County; CDC COVID-19 Investigation Team.  Asymptomatic and presymptomatic SARS-CoV-2 infections in residents of a long-term care skilled nursing facility: King County, Washington, March 2020.   MMWR Morb Mortal Wkly Rep. 2020;69(13):377-381. doi:10.15585/mmwr.mm6913e1 PubMedGoogle ScholarCrossref
    58.
    Kirshblum  SC, DeLauter  G, Lopreiato  MC,  et al.  Screening testing for SARS-CoV-2 upon admission to rehabilitation hospitals in a high COVID-19 prevalence community.   PM R. 2020;12(10):1009-1014. doi:10.1002/pmrj.12454 PubMedGoogle ScholarCrossref
    59.
    Krüger  S, Leskien  M, Schuller  P,  et al.  Performance and feasibility of universal PCR admission screening for SARS-CoV-2 in a German tertiary care hospital.   J Med Virol. 2021;93(5):2890-2898. doi:10.1002/jmv.26770 PubMedGoogle ScholarCrossref
    60.
    Kwon  YS, Park  SH, Kim  HJ,  et al.  Screening clinic for coronavirus disease 2019 to prevent intrahospital spread in Daegu, Korea: a single-center report.   J Korean Med Sci. 2020;35(26):e246. doi:10.3346/jkms.2020.35.e246 PubMedGoogle Scholar
    61.
    LaCourse  SM, Kachikis  A, Blain  M,  et al.  Low prevalence of SARS-CoV-2 among pregnant and postpartum patients with universal screening in Seattle, Washington.   Clin Infect Dis. 2021;72(5):869-872. doi:10.1093/cid/ciaa675PubMedGoogle ScholarCrossref
    62.
    Ladhani  SN, Chow  JY, Janarthanan  R,  et al.  Investigation of SARS-CoV-2 outbreaks in six care homes in London, April 2020.   EClinicalMedicine. 2020;26:100533. doi:10.1016/j.eclinm.2020.100533 PubMedGoogle Scholar
    63.
    Lan  FY, Suharlim  C, Kales  SN, Yang  J.  Association between SARS-CoV-2 infection, exposure risk and mental health among a cohort of essential retail workers in the USA.   Occup Environ Med. 2021;78(4):237-243. PubMedGoogle Scholar
    64.
    Lavezzo  E, Franchin  E, Ciavarella  C,  et al; Imperial College COVID-19 Response Team; Imperial College COVID-19 Response Team.  Suppression of a SARS-CoV-2 outbreak in the Italian municipality of Vo’.   Nature. 2020;584(7821):425-429. doi:10.1038/s41586-020-2488-1 PubMedGoogle ScholarCrossref
    65.
    Livingston  G, Rostamipour  H, Gallagher  P,  et al.  Prevalence, management, and outcomes of SARS-CoV-2 infections in older people and those with dementia in mental health wards in London, UK: a retrospective observational study.   Lancet Psychiatry. 2020;7(12):1054-1063. doi:10.1016/S2215-0366(20)30434-X PubMedGoogle ScholarCrossref
    66.
    Lombardi  A, Consonni  D, Carugno  M,  et al.  Characteristics of 1573 healthcare workers who underwent nasopharyngeal swab testing for SARS-CoV-2 in Milan, Lombardy, Italy.   Clin Microbiol Infect. 2020;26(10):1413.e9-1413.e13. doi:10.1016/j.cmi.2020.06.013PubMedGoogle ScholarCrossref
    67.
    Ly  TDA, Zanini  D, Laforge  V,  et al.  Pattern of SARS-CoV-2 infection among dependant elderly residents living in long-term care facilities in Marseille, France, March-June 2020.   Int J Antimicrob Agents. 2020;56(6):106219. doi:10.1016/j.ijantimicag.2020.106219 PubMedGoogle Scholar
    68.
    Lytras  T, Dellis  G, Flountzi  A,  et al.  High prevalence of SARS-CoV-2 infection in repatriation flights to Greece from three European countries.   J Travel Med. 2020;27(3):taaa054. doi:10.1093/jtm/taaa054 PubMedGoogle Scholar
    69.
    Maechler  F, Gertler  M, Hermes  J,  et al.  Epidemiological and clinical characteristics of SARS-CoV-2 infections at a testing site in Berlin, Germany, March and April 2020-a cross-sectional study.   Clin Microbiol Infect. 2020;26(12):1685.e7-1685.e12. doi:10.1016/j.cmi.2020.08.017PubMedGoogle ScholarCrossref
    70.
    Marossy  A, Rakowicz  S, Bhan  A,  et al.  A study of universal severe acute respiratory syndrome coronavirus 2 RNA testing among residents and staff in a large group of care homes in South London.   J Infect Dis. 2021;223(3):381-388. doi:10.1093/infdis/jiaa565PubMedGoogle ScholarCrossref
    71.
    Marschner  S, Corradini  S, Rauch  J,  et al.  SARS-CoV-2 prevalence in an asymptomatic cancer cohort: results and consequences for clinical routine.   Radiat Oncol. 2020;15(1):165. doi:10.1186/s13014-020-01609-0 PubMedGoogle ScholarCrossref
    72.
    Martinez-Fierro  ML, Ríos-Jasso  J, Garza-Veloz  I,  et al.  The role of close contacts of COVID-19 patients in the SARS-CoV-2 transmission: an emphasis on the percentage of nonevaluated positivity in Mexico.   Am J Infect Control. 2021;49(1):15-20. doi:10.1016/j.ajic.2020.10.002 PubMedGoogle ScholarCrossref
    73.
    Massarotti  C, Adriano  M, Cagnacci  A,  et al.  Asymptomatic SARS-CoV-2 infections in pregnant patients in an Italian city during the complete lockdown.   J Med Virol. 2021;93(3):1758-1760. doi:10.1002/jmv.26458 PubMedGoogle ScholarCrossref
    74.
    Mattar  S, Martinez-Bravo  C, Rivero  R,  et al.  Epidemiological and viral features of a cohort of SARS-CoV-2 symptomatic and asymptomatic individuals in an area of the Colombian Caribbean.   Ann Clin Microbiol Antimicrob. 2020;19(1):58. doi:10.1186/s12941-020-00397-5 PubMedGoogle ScholarCrossref
    75.
    Menting  T, Krause  K, Benz-Tettey  F,  et al.  Low-threshold SARS-CoV-2 testing facility for hospital staff: prevention of COVID-19 outbreaks?   Int J Hyg Environ Health. 2021;231:113653. doi:10.1016/j.ijheh.2020.113653 PubMedGoogle Scholar
    76.
    Migueres  M, Mengelle  C, Dimeglio  C,  et al.  Saliva sampling for diagnosing SARS-CoV-2 infections in symptomatic patients and asymptomatic carriers.   J Clin Virol. 2020;130:104580. doi:10.1016/j.jcv.2020.104580 PubMedGoogle Scholar
    77.
    Milani  GP, Montomoli  E, Bollati  V,  et al; UNICORN Consortium investigators.  SARS-CoV-2 infection among asymptomatic homebound subjects in Milan, Italy.   Eur J Intern Med. 2020;78:161-163. doi:10.1016/j.ejim.2020.06.010 PubMedGoogle ScholarCrossref
    78.
    Nishiura  H, Kobayashi  T, Miyama  T,  et al.  Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19).   Int J Infect Dis. 2020;94:154-155. doi:10.1016/j.ijid.2020.03.020 PubMedGoogle ScholarCrossref
    79.
    Ochiai  D, Kasuga  Y, Iida  M, Ikenoue  S, Tanaka  M.  Universal screening for SARS-CoV-2 in asymptomatic obstetric patients in Tokyo, Japan.   Int J Gynaecol Obstet. 2020;150(2):268-269. doi:10.1002/ijgo.13252 PubMedGoogle ScholarCrossref
    80.
    Olalla  J, Correa  AM, Martín-Escalante  MD,  et al; ROBLE group.  Search for asymptomatic carriers of SARS-CoV-2 in healthcare workers during the pandemic: a Spanish experience.   QJM. 2020;hcaa238.PubMedGoogle Scholar
    81.
    Olmos  C, Campaña  G, Monreal  V,  et al.  SARS-CoV-2 infection in asymptomatic healthcare workers at a clinic in Chile.   PLoS One. 2021;16(1):e0245913. doi:10.1371/journal.pone.0245913 PubMedGoogle Scholar
    82.
    Park  SY, Kim  YM, Yi  S,  et al.  Coronavirus disease outbreak in call center, South Korea.   Emerg Infect Dis. 2020;26(8):1666-1670. doi:10.3201/eid2608.201274 PubMedGoogle ScholarCrossref
    83.
    Park  JH, Jang  JH, Lee  K, Yoo  SJ, Shin  H.  COVID-19 outbreak and presymptomatic transmission in pilgrim travelers who returned to Korea from Israel.   J Korean Med Sci. 2020;35(48):e424. doi:10.3346/jkms.2020.35.e424 PubMedGoogle Scholar
    84.
    Patel  MC, Chaisson  LH, Borgetti  S,  et al.  Asymptomatic SARS-CoV-2 infection and COVID-19 mortality during an outbreak investigation in a skilled nursing facility.   Clin Infect Dis. 2020;71(11):2920-2926. doi:10.1093/cid/ciaa763 PubMedGoogle ScholarCrossref
    85.
    Pavli  A, Smeti  P, Hadjianastasiou  S,  et al.  In-flight transmission of COVID-19 on flights to Greece: an epidemiological analysis.   Travel Med Infect Dis. 2020;38:101882. doi:10.1016/j.tmaid.2020.101882 PubMedGoogle Scholar
    86.
    Petersen  I, Phillips  A.  Three quarters of people with SARS-CoV-2 infection are asymptomatic: analysis of English household survey data.   Clin Epidemiol. 2020;12:1039-1043. doi:10.2147/CLEP.S276825 PubMedGoogle ScholarCrossref
    87.
    Puckett  Y, Wilke  L, Weber  S, Parkes  A, LoConte  NK.  Low rate of SARS-CoV-2 infection in adults with active cancer diagnosis in a nonendemic region in the United States.   WMJ. 2020;119(4):286-288.PubMedGoogle Scholar
    88.
    Ralli  M, Morrone  A, Arcangeli  A, Ercoli  L.  Asymptomatic patients as a source of transmission of COVID-19 in homeless shelters.   Int J Infect Dis. 2021;103:243-245. doi:10.1016/j.ijid.2020.12.031PubMedGoogle ScholarCrossref
    89.
    Rashid-Abdi  M, Krifors  A, Sälléber  A, Eriksson  J, Månsson  E.  Low rate of COVID-19 seroconversion in health-care workers at a Department of Infectious Diseases in Sweden during the later phase of the first wave: a prospective longitudinal seroepidemiological study.   Infect Dis (Lond). 2021;53(3):169-175. doi:10.1080/23744235.2020.1849787PubMedGoogle ScholarCrossref
    90.
    Ren  R, Zhang  Y, Li  Q,  et al.  Asymptomatic SARS-CoV-2 infections among persons entering China from April 16 to October 12, 2020.   JAMA. 2021;325(5):489-492. doi:10.1001/jama.2020.23942 PubMedGoogle ScholarCrossref
    91.
    Rincón  A, Moreso  F, López-Herradón  A,  et al.  The keys to control a COVID-19 outbreak in a haemodialysis unit.   Clin Kidney J. 2020;13(4):542-549. doi:10.1093/ckj/sfaa119 PubMedGoogle ScholarCrossref
    92.
    Roxby  AC, Greninger  AL, Hatfield  KM,  et al.  Outbreak investigation of COVID-19 among residents and staff of an independent and assisted living community for older adults in Seattle, Washington.   JAMA Intern Med. 2020;180(8):1101-1105. doi:10.1001/jamainternmed.2020.2233 PubMedGoogle ScholarCrossref
    93.
    Sacco  G, Foucault  G, Briere  O, Annweiler  C.  COVID-19 in seniors: findings and lessons from mass screening in a nursing home.   Maturitas. 2020;141:46-52. doi:10.1016/j.maturitas.2020.06.023 PubMedGoogle ScholarCrossref
    94.
    Santos  EJF, Ferreira  RJO, Batista  R,  et al.  Health care workers not in the frontline are more frequently carriers of coronavirus disease 2019: the experience of a tertiary Portuguese hospital.   Infect Prev Pract. 2020;2(4):100099. doi:10.1016/j.infpip.2020.100099 PubMedGoogle Scholar
    95.
    Scheier  T, Schibli  A, Eich  G,  et al.  Universal admission screening for SARS-CoV-2 infections among hospitalized patients, Switzerland, 2020.   Emerg Infect Dis. 2021;27(2):404-410. doi:10.3201/eid2702.202318 PubMedGoogle ScholarCrossref
    96.
    Shah  AS, Walkoff  LA, Kuzo  RS,  et al.  The utility of chest computed tomography (CT) and RT-PCR screening of asymptomatic patients for SARS-CoV-2 prior to semiurgent or urgent hospital procedures.   Infect Control Hosp Epidemiol. 2020;41(12):1375-1377. doi:10.1017/ice.2020.331 PubMedGoogle ScholarCrossref
    97.
    Shi  SM, Bakaev  I, Chen  H, Travison  TG, Berry  SD.  Risk factors, presentation, and course of coronavirus disease 2019 in a large, academic long-term care facility.   J Am Med Dir Assoc. 2020;21(10):1378-1383.e1. doi:10.1016/j.jamda.2020.08.027 PubMedGoogle ScholarCrossref
    98.
    Singer  JS, Cheng  EM, Murad  DA,  et al.  Low prevalence (0.13%) of COVID-19 infection in asymptomatic pre-operative/pre-procedure patients at a large, academic medical center informs approaches to perioperative care.   Surgery. 2020;168(6):980-986. doi:10.1016/j.surg.2020.07.048 PubMedGoogle ScholarCrossref
    99.
    Tang  H, Tian  JB, Dong  JW,  et al.  Serologic detection of SARS-CoV-2 infections in hemodialysis centers: a multicenter retrospective study in Wuhan, China.   Am J Kidney Dis. 2020;76(4):490-499.e1. doi:10.1053/j.ajkd.2020.06.008 PubMedGoogle ScholarCrossref
    100.
    Tang  O, Bigelow  BF, Sheikh  F,  et al.  Outcomes of nursing home COVID-19 patients by initial symptoms and comorbidity: results of universal testing of 1970 residents.   J Am Med Dir Assoc. 2020;21(12):1767-1773.e1. doi:10.1016/j.jamda.2020.10.011 PubMedGoogle ScholarCrossref
    101.
    Temkin  E; Healthcare Worker COVID-19 Surveillance Working Group.  Extremely low prevalence of asymptomatic COVID-19 among healthcare workers caring for COVID-19 patients in Israeli hospitals: a cross-sectional study.   Clin Microbiol Infect. 2021;27(1):130.e1-130.e4. doi:10.1016/j.cmi.2020.09.040PubMedGoogle ScholarCrossref
    102.
    Trahan  MJ, Mitric  C, Malhamé  I, Abenhaim  HA.  Screening and testing pregnant patients for SARS-CoV-2: first-wave experience of a designated COVID-19 hospitalization centre in Montreal.   J Obstet Gynaecol Canada. 2021;43(5):571-575. doi:10.1016/j.jogc.2020.11.001 PubMedGoogle Scholar
    103.
    Tsou  TP, Chen  WC, Huang  ASE, Chang  SC; Taiwan COVID-19 Outbreak Investigation Team.  Epidemiology of the first 100 cases of COVID-19 in Taiwan and its implications on outbreak control.   J Formos Med Assoc. 2020;119(11):1601-1607. doi:10.1016/j.jfma.2020.07.015 PubMedGoogle ScholarCrossref
    104.
    van Buul  LW, van den Besselaar  JH, Koene  FMHPH, Buurman  BM, Hertogh  CMPM; COVID-19 NH-Study Group; COVID-19 NH-Study Group.  Asymptomatic cases and limited transmission of SARS-CoV-2 in residents and healthcare workers in three Dutch nursing homes.   Gerontol Geriatr Med. 2020;6:2333721420982800. doi:10.1177/2333721420982800 PubMedGoogle Scholar
    105.
    Varnell  C  Jr, Harshman  LA, Smith  L,  et al.  COVID-19 in pediatric kidney transplantation: the Improving Renal Outcomes Collaborative.   Am J Transplant. 2021;21(8):2740-2748. doi:10.1111/ajt.16501 PubMedGoogle ScholarCrossref
    106.
    Wadhwa  A, Fisher  KA, Silver  R,  et al.  Identification of presymptomatic and asymptomatic cases using cohort-based testing approaches at a large correctional facility: Chicago, Illinois, USA, May 2020.   Clin Infect Dis. 2021;72(5):e128-e135. doi:10.1093/cid/ciaa1802PubMedGoogle ScholarCrossref
    107.
    Wi  YM, Lim  SJ, Kim  SH,  et al.  Response system for and epidemiological features of COVID-19 in Gyeongsangnam-do Province in South Korea.   Clin Infect Dis. 2021;72(4):661-667. PubMedGoogle ScholarCrossref
    108.
    Wood  J, Datta  D, Hudson  BL,  et al.  Prevalence of asymptomatic SARS-CoV-2 infection in children and adults in Marion County, Indiana.   Cureus. 2020;12(8):e9794. doi:10.7759/cureus.9794PubMedGoogle Scholar
    109.
    Yamahata  Y, Shibata  A.  Preparation for quarantine on the cruise ship Diamond Princess in Japan due to COVID-19.   JMIR Public Health Surveill. 2020;6(2):e18821. doi:10.2196/18821 PubMedGoogle Scholar
    110.
    Yassa  M, Yirmibes  C, Cavusoglu  G,  et al.  Outcomes of universal SARS-CoV-2 testing program in pregnant women admitted to hospital and the adjuvant role of lung ultrasound in screening: a prospective cohort study.   J Matern Fetal Neonatal Med. 2020;33(22):3820-3826. doi:10.1080/14767058.2020.1798398 PubMedGoogle ScholarCrossref
    111.
    Yau  K, Muller  MP, Lin  M,  et al.  COVID-19 outbreak in an urban hemodialysis unit.   Am J Kidney Dis. 2020;76(5):690-695.e1. doi:10.1053/j.ajkd.2020.07.001 PubMedGoogle ScholarCrossref
    112.
    Yousaf  AR, Duca  LM, Chu  V,  et al.  A prospective cohort study in nonhospitalized household contacts with severe acute respiratory syndrome coronavirus 2 infection: symptom profiles and symptom change over time.   Clin Infect Dis. 2021;73(7):e1841-e1849. doi:10.1093/cid/ciaa1072PubMedGoogle ScholarCrossref
    113.
    Zhang  T, Wu  HT, Wang  LH, Yang  WZ.  Scenario-based study of medical resource requirement rapid assessment under the COVID-19 pandemic  [in Chinese].  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(0):E059. doi:10.3760/cma.j.cn112338-20200401-00488PubMedGoogle Scholar
    114.
    Zhang  S, Guo  M, Wu  F,  et al.  Factors associated with asymptomatic infection in health-care workers with severe acute respiratory syndrome coronavirus 2 infection in Wuhan, China: a multicentre retrospective cohort study.   Clin Microbiol Infect. 2020;26(12):1670-1675. doi:10.1016/j.cmi.2020.08.038 PubMedGoogle ScholarCrossref
    115.
    Zhao  D, Wang  M, Wang  M,  et al.  Asymptomatic infection by SARS-CoV-2 in healthcare workers: a study in a large teaching hospital in Wuhan, China.   Int J Infect Dis. 2020;99:219-225. doi:10.1016/j.ijid.2020.07.082 PubMedGoogle ScholarCrossref
    116.
    Black  JRM, Bailey  C, Przewrocka  J, Dijkstra  KK, Swanton  C.  COVID-19: the case for health-care worker screening to prevent hospital transmission.   Lancet. 2020;395(10234):1418-1420. doi:10.1016/S0140-6736(20)30917-X PubMedGoogle ScholarCrossref
    117.
    Passarelli  VC, Faico-Filho  K, Moreira  LVL,  et al.  Asymptomatic COVID-19 in hospital visitors: the underestimated potential of viral shedding.   Int J Infect Dis. 2021;102:412-414. doi:10.1016/j.ijid.2020.10.057 PubMedGoogle ScholarCrossref
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