Clinical Characteristics and Transmission of COVID-19 in Children and Youths During 3 Waves of Outbreaks in Hong Kong

IMPORTANCE Schools were closed intermittently across Hong Kong to control the COVID-19 outbreak, which led to significant physical and psychosocial problems among children and youths. OBJECTIVE To compare the clinical characteristics and sources of infection among children and youths with COVID-19 during the 3 waves of outbreaks in Hong Kong in 2020. DESIGN, SETTING, AND PARTICIPANTS This cross-sectional study involved children and youths aged 18 years or long-term complications that might develop after the primary infection with COVID-19. Our study transmission of SARS-CoV-2 within school campuses in Hong Kong rare, of whether the schools were closed or reopened. 3 individuals with domestic COVID-19 infections in our reported to schoolmates in close contact. There were no reports that teachers had transmitted COVID-19 to their students. most of the pediatric patients imported infections in the first and second waves. They students returning Hong Kong overseas boarding schools infected schoolmates. many their learning online physical social myopia progression in children during home pre-COVID-19 et closuresinHongKongwereassociatedwithsignificantchangesinthephysicalandpsychosocialwell-being children, and the differences in outcomes were among families of children with special educational needs, families with members who had mental health problems, single-parent families, and families with lower incomes. School closures at critical periods in the educational of primary school children also associated with deleterious outcomes, such as educational attainment and life expectancies. 21,22 The Hong Kong of May attended by more than 50000 secondary school students in person. Candidates were not allowed to attend if they undergoing mandatory quarantine. Examination venues were disinfected regularly and had adequate ventilation. Candidates’ temperatures were measured before they could enter the examination hall, and they were required to maintain social distancing of at least 1.8 m and wear masks during the examination. There were no reports of students being infected during the examinations. 23 In addition, the sources of infection for most individuals in our study group were family members and not schools, and it is typical in Hong Kong that extended family members reside in the same household.


Introduction
In the COVID-19 outbreak, caused by SARS-CoV-2, more than 111 million people have been infected and more than 2.4 million individuals have died worldwide. 1 The first COVID-19 patient in Hong Kong was diagnosed on January 23, 2020. 2 Hong Kong is now experiencing its fourth wave of COVID-19 outbreaks, during which multiple public health policies have been implemented to facilitate social distancing to reduce the spread of COVID-19. 3 These public health policies have had a far greater impact than expected, and nearly all children and youths with COVID-19 in Hong Kong have had only mild illness. 4,5 During the 4 COVID-19 outbreaks, the Government of Hong Kong implemented territory-wide school closures intermittently. 6,7 During each school closure, face-to-face teaching was replaced by homeschooling and online classes. Students attended classroom lessons for less than 3 months in 2020. A 2020 large-scale local study 8 found that the prolonged school closures may have been associated with increased risk among children of developing psychosocial problems, which were associated with decreased emotional and social functioning and decreased physical activity levels. Existing inequalities, such as having children with special educational needs, in families with increased risk of psychosocial problems were associated with increased levels of these adverse outcomes. A 2020 study 9 in the US found that SARS-CoV-2 infection in children and youths was not associated with attending school or childcare centers, and a 2021 study 10 in the US found that secondary transmission of SARS-CoV-2 within schools was rare. However, evidence concerning the COVID-19 outbreaks in schools in Hong Kong is lacking. Furthermore, To et al 11

Methods
The ethics committees of all 7 Hong Kong Hospital Authority clusters approved this cross-sectional study and waived individual informed consent given that the data were collected retrospectively and anonymously. This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.
The study included children and youths aged 18 years or younger with COVID-19 confirmed by the positive detection of SARS-CoV-2 in respiratory specimens by reverse transcriptase polymerase chain reaction (RT-PCR) from January 23 through December 2, 2020. Surveillance criteria during the 3 waves of outbreaks were identical. These children and youths received SARS-CoV-2 RT-PCR testing because they were symptomatic, were close contacts of an individual with a confirmed infection, or were returned international travelers receiving screening at the border. All children and youths with confirmed COVID-19, including those without symptoms, were admitted into the pediatric isolation wards of public hospitals in Hong Kong as part of the mandatory territory-wide infectioncontrol policy.  of the territory-wide pediatric COVID-19 study. The deidentified information was submitted to a centralized electronic data-collection system for analysis . The collected data included sex, age at   diagnosis, clinical symptoms, dates of admission and discharge, travel history prior to diagnosis of   COVID-19 infection, identifiable contacts with individuals with confirmed COVID-19 infection, and COVID-19-related complications after initial discharge.

Definitions
We defined the periods of the 3 waves of outbreaks as follows: the first wave was from January 23 through March 21, 2020; the second wave was from March through July 4, 2020; and the third wave was from July 5 through December 2, 2020. All individuals were classified as having domestic infections (ie, with no recent international travel history) or imported infections (ie, with recent travel history outside of the Hong Kong Special Administrative Region and Mainland China within 2 weeks prior to symptom onset). The hospital length of stay was defined as the period from admission to discharge. Patients discharged before July 6, 2020, were required to have 2 negative nasopharyngeal swab SARS-CoV-2 RT-PCR tests taken at least 24 hours apart before being discharged from the hospital. Patients testing positive had repeat RT-PCR tests every 2 to 3 days until the nasopharyngeal swab results were negative. The discharge criterion was modified on July 6, 2020, to any detectable SARS-CoV-2 antinucleoprotein immunoglobulin G (IgG) antibodies in the serum regardless of a positive respiratory specimen RT-PCR result.

Statistical Analysis
Statistical analyses were performed using SPSS statistical software version 26 (IBM) from December 2020 through January 2021. We used χ 2 exact test to compare differences in clinical symptoms.
One-way analysis of variances was used to detect differences in the mean age at diagnosis and mean length of hospital stay among patients in the 3 waves of outbreaks. The hospital admissions during the study period were presented as plots by admission dates. A 2-tailed P value less than .05 was considered statistically significant.

Discussion
To our knowledge, this cross-sectional study is the first study comparing the clinical characteristics and transmission patterns among children and youths with COVID-19 in East Asia in 2020. We found  11 Children and youths presenting with diarrhea were more likely to report having contact with more than 2 infected individuals, which could be associated with the exposure to a higher viral load, given that a previous study found a higher viral load in the stools of patients with diarrhea 14 This study also identified 3 patients with rare manifestations of COVID-19. Chilblains, also known as COVID toes, presenting in both children and adults is a dermatological manifestation recognized as a symptom of COVID-19. 15 Although the exact pathophysiology of chilblains in individuals with COVID-19 remains to be elucidated, possible mechanisms include an abnormal inflammatory response attributed to type I interferonopathies or thrombotic microvasculature induced by complement activation and procoagulation state. 16  if they were undergoing mandatory quarantine. Examination venues were disinfected regularly and had adequate ventilation. Candidates' temperatures were measured before they could enter the examination hall, and they were required to maintain social distancing of at least 1.8 m and wear masks during the examination. There were no reports of students being infected during the examinations. 23 In addition, the sources of infection for most individuals in our study group were family members and not schools, and it is typical in Hong Kong that extended family members reside in the same household. Students under quarantine at home could be supported by online learning.

Limitations
The study has several limitations, and data from this study should be interpreted with the following caveats. Although there were 488 patients confirmed with COVID-19 infection by the end of the third wave, some patients in the third wave had not yet been discharged from isolation facilities and were not yet captured in our database.
We have yet to include the clinical characteristics of the individuals' family members who also had COVID-19. Hong Kong is a unique city in the Southeast Asia, with many travelers coming from overseas, as well as tens of thousands of Hong Kong children and youths studying abroad and returning to Hong Kong several times per year. This could make it difficult to generalize our findings to other cities in China and Asia. Additionally, the number of patients in the first wave was small. Nevertheless, we have included all Hong Kong children and youths with COVID-19 in the first and second waves, and the criteria implemented in Hong Kong for the screening of individuals without symptoms who were carriers and of close contacts of individuals who were infected remained the same throughout the 3 waves of outbreak.

Conclusions
In this cross-sectional study, children and youths with COVID-19 had a wide range of clinical presentations, from no symptoms to postinfectious immune-mediated complications. Recent reports found that a new SARS-CoV-2 strain, the variant of concern 202012/01, has emerged in the United Kingdom, which is likely to be more transmissible than existing circulating strains. 31 Whether this new circulating strain will lead to a massive increase in global transmissions or to a new clinical spectrum of diseases remains to be investigated. Nevertheless, our study found that nearly all children and youths with COVID-19 were infected through household transmission. Risk of being infected at school was very small if social distancing and face mask policies were enforced. Given that