Table. Characteristics of Included Studies.

Source	Status	Location	Recruitment of index cases	Recruitment and isolation of contacts	Contact type	No. of clusters, index cases, and contacts	Case definition and virus testing	Child/adolescent age; adult age
Contact-tracing studies
Zhang et al,10 2020	Published and peer reviewed	Hunan province, China	All confirmed cases identified by Hunan Center for Disease Control between January 16 and March 1, 2020	Close contacts were identified through contact tracing of confirmed cases and placed under medical observation for 14 d. A close contact was defined as an individual who had unprotected close contact (within 1 m) with a confirmed case or an asymptomatic infection within 2 d before their symptom onset or sample collection.	All contact types	114 Clusters representing 136 index cases and 7193 contact cases. 1 Index case (0.7%) was <15 y.	Positive findings on RT-PCR. All close contacts were tested in accordance with local policy regardless of symptoms. Percentage of contacts tested was not stated.	0-14 y; ≥15 y
Li et al,16 2020	Published and peer reviewed	Hubei province, China (hospitals in Zaoyang City and Chibi City)	Index cases identified from 2 hospitals from January 1 to February 13, 2020. Index cases were excluded if members of their family had links to Wuhan province. Not clear if all cases from hospital were sampled or just a subset.	All household contacts were quarantined immediately for 14 d by the local government and monitored daily.	Household contacts	105 Index patients with their households and all family contacts (n = 392). The proportion of index cases who were children was not reported.	Positive findings on RT-PCR. Nasopharyngeal swab samples were collected at the beginning and the middle of quarantine. All contacts were tested 2 to 4 times.	0-17 y; ≥18 y
Cheng et al,17 2020	Published and peer reviewed	Taiwan	The initial 100 confirmed cases in Taiwan between January 15 and March 18, 2020.	Close contacts were identified through epidemiological investigation and defined as a person who did not wear appropriate personal protection equipment while having face-to-face contact with a confirmed case for more than 15 min during the investigation period (defined by epidemiological investigation and typically up to 4 d prior to symptom onset or test date for asymptomatic cases). All close contacts were quarantined at home for 14 d after their last exposure to the index case.	All contact types	100 Index cases and 2761 close contacts. The youngest index case was aged 11 y, although the proportion of index cases that were children was not reported.	Positive findings on RT-PCR. Routine testing for household and health care worker contacts (30.7%). Other contacts (69.3%) were only tested if symptomatic.	0-19 y; ≥20 y
Wang et al,18 2020	Published and peer reviewed	Wuhan province, China	Patients hospitalized in Union Hospital (n = 85) on February 13 and 14, 2020. Not clear if all cases from hospital were sampled or just a subset.	Household contacts of the hospitalized patients, observed for 14 d.	Household contacts	85 Households corresponding to the 85 patients were enrolled and 155 household contacts.	Positive findings on RT-PCR and throat swabs. Process for testing household members was not stated, but 33% of household contacts were not tested for SARS-CoV-2.	Child age not defined.
Mizumoto et al,19 2020	Preprint	Japan	Cases that were domestically acquired and confirmed by RT-PCR by March 7, 2020.	Contacts of index cases, definition, and method of ascertainment not given. No details on isolation of contacts.	Not stated. The total number of contacts (8 per index case) suggests these are likely all contacts.	313 Cases and their 2496 close contacts.	Positive findings on RT-PCR. Process and eligibility for testing of contacts not described.	0-19 y; ≥20 y
Wang et al,20 2020	Published and peer reviewed	Beijing, China	All laboratory-confirmed (ie, RT-PCR–confirmed) cases in Beijing up to February 21, 2020, recruited through Beijing Center for Disease Control.	From February 28 to March 8, 2020, all household members of index cases were observed for 14 d. Testing and quarantine of contacts not clearly defined.	Household contacts	124 of 137 Eligible families participated. No primary cases were <18 y.	Index and secondary cases defined as positive findings on RT-PCR. Proportion of PCR testing of secondary contacts was not stated.	0-17 y; ≥18 y
Park et al,12 2020	Published and peer reviewed	South Korea	All laboratory-confirmed cases in Korea registered with the Korea Center for Disease Control from January 20 to May 13, 2020.	All contacts of index cases registered with Korea Center for Disease Control through a comprehensive national contact-tracing system and observed for a mean of 10 d.	Household and nonhousehold contacts. Only data on household contacts were included in this review.	Studied 59 073 contacts (10 592 were household contacts) of 5706 index cases. Only index cases who reported 1 or more contacts were included; however, only 52% of 10 962 national cases reported in the period were included.	Household and health care worker contacts routinely tested by RT-PCR. Other contacts only tested if symptomatic.	0-19 y; ≥20 y
Dattner et al,21 2020	Preprint	Israel	Identification of all households in the city of Bnei Brak, Israel, where all household members had been tested by PCR and 1 or more members had positive findings. Households identified through the Israeli COVID-19 database until May 2, 2020, were included.	All household members included. A total of 51% of population were <20 y.	Household	637 Houses comprising 3353 people, of whom 1510 were positive for SARS-CoV-2. All eligible households were included. Figures 2, 3, and 4 were derived from supplied estimated probabilities of children or adults being the index.	Positive findings on RT-PCR testing of all household members, including index cases and contacts.	0-19 y; ≥20 y
Hu et al,22 2020	Preprint	Hunan province, China	All cases with contact details were identified from the notifiable infectious diseases reporting system in Hunan province from January 16 to April 2, 2020.	Contacts were quarantined for 14 d and tested with PCR at least once during quarantine. After February 7, 2020, all contacts were tested, but only symptomatic contacts were tested before (approximately 50% of contacts tested).	All contacts	1178 Cases and their 15 648 contacts.	Positive findings on RT-PCR.	0-19 y; ≥20 y
Laxminarayan et al,23 2020	Preprint	Tamil Nadu and Andhra Pradesh, India	Index cases identified from state registries and contacts traced by public health agencies in each state from March 5 to June 4, 2020, in Tamil Nadu, India, and from March 5 to May 29, 2020, in Andhra Pradesh, India.	Contacts traced by public health agencies and tested between 5 and 15 d of exposure. Insufficient detail provided. Note that there were twice as many close contacts per index case <18 y compared with >18 y.	All contacts	4206 Confirmed cases and 64 031 contacts. Note only 4206 cases included of 33 584 total cases (13%), with no detail given on nonrecruitment.	Positive findings on RT-PCR of all contacts regardless of symptoms.	0-17 y; ≥18 y
Liu et al,24 2020	Published and peer reviewed	Guandong province, China	All cases identified by intensive regional surveillance by local center for disease control from January 15 to March 15, 2020.	Contacts traced and monitored with PCR from throat swabs taken every few days for 14 d; 84% of contacts were quarantined in centralized stations.	All contacts	1361 Cases reported and 11 868 contacts traced and quarantined. Analysis included 11 580 contacts (98%).	Positive findings on RT- PCR from throat swabs.	0-19 y; ≥20 y
Rosenberg et al,25 2020	Published and peer reviewed	New York State excluding New York City	Identified and studied 229 initially confirmed cases by PCR from March 2 to 12, 2020.	Active contact tracing by county and state health departments. All household contacts were eligible for PCR testing. Contacts tested 0 to 10 d after index case (43% were tested on day 0, ie, initial index diagnosis day).	Household	229 Index cases and 343 household contacts.	Positive findings on RT-PCR. All household contacts were eligible for PCR testing; however, not stated what proportion were tested.	0-17 y; ≥18 y
Yousaf et al,26 2020	Published and peer reviewed	Milwaukee, Wisconsin, and Salt Lake City, Utah	All PCR-positive cases from 2 cities were identified through routine public health surveillance and recruited between March 22 and April 22, 2020.	Active contact tracing by public health departments. All contacts were observed for 14 d with 2 swab tests (RT-PCR) on day 0 and day 14, plus if symptomatic.	Household	195 of 198 Contacts participated (98.5%). Numbers of index cases not stated.	Positive findings on RT-PCR. All household contacts tested.	0-17 y; ≥18 y
Chaw et al,27 2020	Preprint	Brunei	All 71 initial cases in Brunei, which arose following a religious event, with cases detected after March 9, 2020.	Detailed contact tracing by Ministry of Health, with RT-PCR testing of all reported contacts. All contacts were quarantined for 14 d and retested if symptomatic.	All contacts	71 Index cases and 1755 close contacts. All contacts participated.	Positive findings on RT-PCR.	0-19 y; ≥20 y
van der Hoek et al,7,28 2020	Published and peer reviewed	The Netherlands	National surveillance data from 2 Dutch systems: (1) OSIRIS, a registry of all laboratory-confirmed cases and (2) HPZone. Data on contact tracing from 23 of 25 Dutch municipalities.	Data included up to April 2, 2020. Contact tracing was undertaken for all cases registered in HPZone. Contact infection status identified through linkage to the main national surveillance database, suggesting that only symptomatic secondary cases were included.	All contacts	231 Cases and 709 close contacts. Proportion of contacts tested not stated.	Positive findings on RT-PCR.	<19 y
School contact-tracing studies
Macartney et al,29 2020	Published and peer reviewed	New South Wales, Australia	COVID-19 cases in 25 educational settings (15 schools and 10 early learning centers) for which a person (student or staff) with proven COVID-19 (positive findings on PCR) had attended while infectious. Identified through state Notifiable Conditions Information Management System. Schools remained open but students dismissed from March 23, 2020 (<5% student attendance). Note that school attendance remained high at the time that secondary cases were identified in schools, and early-years settings did not close.	January 25 to April 9, 2020. All close contacts (a person who has been in face-to-face contact for at least 15 min or in the same room for at least 40 min with a case while infectious) were followed up. All close contacts observed and tested if symptomatic during the 14-d isolation period. 7 Settings had testing of all contacts 5 to 10 d after last contact plus serology after day 21.	Contacts in educational setting only	27 Primary cases (12 student and 15 staff cases) and their 1448 school-related and early learning–related close contacts from 25 educational settings. 12 High school cases (8 students; 4 staff) from 10 schools had a total of 695 contacts (598 students; 97 staff). The 5 primary school cases (1 student; 4 staff) from 5 schools had a total of 218 contacts (179 student; 39 staff). 1448 Contacts identified; 663 (43.5%) were tested (PCR or serology or both).	Positive findings on RT-PCR or serology (specific IgG, IgA, IgM detection using indirect immunofluorescence). Swabs taken from 542 of 1448 contacts (37.4%). Serology was performed in 208 of 1448 contacts (14.3%).	6 wk-18 y; ≥20 y
Heavey et al,30 2020	Published and peer reviewed	Ireland	Screened Ireland national surveillance to identify all PCR-positive cases in children or adults who attended school settings from March 1 to March 12, 2020.	Contacts traced and advised to quarantine at home for 14 d. Tested (PCR) only if symptomatic.	All contacts, including school contacts	6 Index cases identified (3 adult; 3 <18 y). 1155 Contacts identified (924 children; 101 adults).	Positive findings on RT-PCR testing if symptomatic.	0-17 y; ≥18 y
Yung et al,31 2020	Published and peer reviewed	Singapore	3 Potential SARS-CoV-2 seeding incidents in educational settings in Singapore identified from national surveillance during February and March 2020.	Close school contacts (eg, classmates) quarantined for 14 d. Contacts in 1 school and 1 preschool were tested only if symptomatic; these schools were not closed. Contacts in 1 preschool were tested by PCR after an outbreak causing school closure.	School contacts only	3 PCR-positive child index cases were identified from 2 preschools and 1 secondary school. 188 Contacts were studied, of whom 119 (63%) were tested.	Positive findings on RT-PCR.	1-16 y
Population screening studies
Gudbjartsson et al,32 2020	Published and peer reviewed	Iceland	First infection diagnosed on February 28, 2020; containment measures put in place. Primary schools open but some secondary schools closed and moderate restrictions on social contacts from March 13 to April 6, 2020.	National population screening. Open invitation for 87% of participants through online portal but with collection of sample from 1 location (Reyjkavik) and random invitation for a subsample (13%). Children <10 y made up 6.4% of sample. Participation in the study was primarily by request of participants rather than by random sampling, which may have introduced biases in participation.	NA	Only population screening sample reported here.	Positive findings on RT-PCR on nasopharyngeal and oropharyngeal samples.	0-9 y; ≥10 y
Lavezzo et al,33 2020	Published and peer reviewed	Vo, Veneto region, Italy	Quarantined community in an area of Italy that was affected early and severely in the epidemic; area was locked down starting February 23, 2020, for 2 weeks. Study undertaken close to the imposition of very strict social distancing measures in the region.	All age groups were homogeneously sampled, with age-specific percentages ranging from 70.8% to 91.6%. 2 Surveys undertaken; first survey only included here (overall response rate 85.9%). Those <21 y made up 17% of sample and had a participation rate of 94% (0-10 y) and 95% (11-20 y).	NA	We present data only from this first survey, although the article also reports a second survey undertaken during lockdown.	Positive findings on RT-PCR on nasopharyngeal samples.	0-20 y; ≥21 y
Public Health Agency of Sweden,34 2020	Online report	Sweden	First death reported in Stockholm on March 11, 2020. Voluntary social distancing measures recommended starting March 16, 2020, with secondary schools recommended to teach virtually. Primary schools and early-years settings remained open throughout.	2 Nationally representative surveys undertaken by the Public Health Agency of Sweden. Participants invited by email: 2571 of 4480 (57%) participated in April and 2957 of 4487 (66%) in May. Children 0-15 y made up 18.9% of the April sample and 17.2% of the May sample. Participants performed home self-sampling using nasopharyngeal swabs.	NA	NA	Positive findings on RT-PCR on nasopharyngeal samples.	0-15 y; ≥16 y
Office for National Statistics,35 2020	Online report	England	Strict national social distancing measures enacted March 20, 2020, with gradual easing of lockdown starting May 25, 2020.	Representative sample of 35 801 individuals in England. Those aged 2-19 y made up 17% of the population. Cases were identified by home self-sampling using nasopharyngeal swabs, with carers swabbing young children. 79% of Invited participants provided 1 or more swabs.	NA	Repeated surveys carried out each week. Data shown here are the cumulative prevalence of those ever positive between April 26 and June 27, 2020.	Positive findings on RT-PCR on nasopharyngeal samples.	2-19 y; ≥20 y
Pollán et al,36 2020	Online report	Spain	Strict social distancing was imposed on March 14, 2020. Some restrictions were lifted on April 27, 2020, and further restrictions lifted on May 11, 2020.	Undertaken by Spanish Ministry of Health. National representative sample obtained from random sampling of households in municipalities across Spain. Of 102 803 approached, 61 075 participants provided point of care samples and 51 958 included in both immunoassay and point of care. Those aged 0-19 y (n = 11 464) made up 23% of the point of care sample and 12.6% of the immunoassay sample.	NA	We used the point-of-care data here owing to the sample being representative of the child population, unlike the immunoassay test.	Point-of-care test: rapid immunochromatography IgG (Zhejiang Orient Gene Biotech) and SARS-CoV-2 immunoassay (Abbott Laboratories). Comparison of the rapid test IgG with SARS-CoV-2 serology in 16 953 of the study sample found 97.3% agreement between tests.	0-19 y; ≥20 y
National Institute for Public Health and the Environment,7,37 2020	Online report	The Netherlands	Social distancing measures introduced gradually from March 11, 2020. Schools closed from March 15, 2020.	Undertaken by the Netherlands National Institute for Public Health and the Environment. Population-based sampling was undertaken in a random sample of a randomly chosen subset of municipalities across the Netherlands. Total sample of 2096. Those <20 y made up 20% of sample.	NA	Data provided by author F. v. d. K.	Positive findings on serology (IgG).	0-19 y; ≥20 y
Hallal et al,38 2020	Preprint	Brazil	First cases reported February 27, 2020, with local/state lockdowns during March and April 2020. Some states began to relax measures in April 2020.	Nationwide seroprevalence survey in 133 sentinel cities in 26 Brazilian states. Randomly selected households visited and finger-prick rapid serology test used. Total sample was 24 995 with household response rate (55%). Children heavily underrepresented—2.2% were aged 0-9 y and 9.1% were aged 10-19 y.	NA	NA	Rapid lateral flow test used in our analysis (Wondfo SARS-CoV-2).	0-19 y
Shakiba et al,39 2020	Preprint	Iran	Population-based seroprevalence study in 5 counties in Guilan province, Iran, in April 2020—previously very high virus prevalence.	Multistage cluster random sampling approach and telephone recruitment of head of household. 196 of 632 Approached households participated (31%) with 528 participants.	NA	NA	VivaDiag COVID-19 IgM/IgG serology.	0-17 y; ≥18 y
Biggs et al,40 2020	Published and peer reviewed	Georgia	Study undertaken by US Centers for Disease Control and Prevention to coincide with end of shelter-in-place orders (April 3 to 30, 2020).	Survey of a random sample of households in 2 metropolitan Atlanta countries. 696 Persons from 394 of 1675 households (23.5%) participated. Children <18 y were 6.9% of sample compared with 22.4% of population.	NA	NA	Total antibody measured using VITROS 3600 Immunodiagnostic System (Ortho Clinical Diagnostics).	0-17 y; ≥18 y
Stringhini et al,41 2020	Published and peer reviewed	Geneva canton, Switzerland	First case on February 26, 2020. Schools closed on March 16, 2020, and strict social distancing measures introduced March 20, 2020. Seroprevalence initiated using a population-based sample in canton.	Population based but not fully random sample within canton (region). 1300 Randomly selected adults approached each week for 5 weeks and invited to bring all household members ≥5 y for serology. Only nonsymptomatic individuals studied. A total of 2766 of 5492 (50.4%) agreed to participate, and data presented here for first 1360. Total of 16.4% of sample aged 0-19 y, similar to population.	NA	Indeterminate cases were treated as having negative findings in calculating data for the meta-analysis.	ELISA to spike protein (Euroimmun).	5-19 y; ≥20 y
Nawa et al,42 2020	Preprint	Utsunomiya, Japan	First cases in Japan starting January 15, 2020. All schools closed February 27, 2020. Survey conducted between the first and second spikes of infection in the city.	Population-based seroprevalence survey: a random sample of 1000 households approached. A total of 742 of 2290 persons (32%) participated. 13% Were <18 y, similar to population.	NA	NA	IgG (Shenzhen YHLO Biotech).	0-17 y; ≥18 y
Pagani et al,43 2020	Preprint	Lombardy, Italy	The town of Castiglione d’Adda, 4550 inhabitants had high numbers of infections from early in the pandemic. Local lockdown occurred from February 23, 2020.	Entire population (all ages) invited to participate; recruited 4174 of 4550 inhabitants (92%) who had rapid capillary testing, of whom a random sample of 562 (stratified for age and sex) had formal serology by venipuncture. Those <19 y made up 12% of the rapid and formal serology samples.	NA	22% of Population showed overall positivity (22.2% on rapid test, 22.6% on formal serology). Rapid test used in meta-analyses here, as findings from formal serology were highly similar.	Rapid capillary testing: lateral-flow immunochromatographic test (Prima Lab); serology: CLIA, IgG anti–SARS-CoV-2 (Abbott Laboratories).	0-19 y; ≥20 y
Weis et al,44 2020	Preprint	Thuringia, Germany	Seroprevalence survey in the previously quarantined community Neustadt-am-Rennsteig, from 6 weeks after a SARS-CoV-2 outbreak (March 22, 2020). Local lockdown initiated.	All community households invited. A total of 626 of 883 (71% of community) enrolled. Focus on child participation and blood collection to be representative. Children 1-17 y were 9.5% of the sample; 620 gave blood and 600 participants had all 6 serological tests performed.	NA	NA	Serology by 6 quantification methods: 2 ELISA and 4 immunoassay: EDI Novel Coronavirus SARS-CoV-2 IgG ELISA kit (Epitope Diagnostics); SARS-CoV-2 IgG ELISA kit (Euroimmun); SARS-CoV-2 S1/S2 IgG CLIA kit (DiaSorin); 2019-nCoV IgG kit (Snibe Co); SARS-CoV-2 IgG CMIA kit (Abbott Laboratories); and Elecsys Anti-SARS-CoV-2 kit (Roche).	1-17 y; ≥18 y
Streeck et al,45 2020	Preprint	Gangelt, Germany	Carnival held on February 15, 2020. Strict local social distancing measures introduced on February 28, 2020, due to local outbreak and deaths.	A random sample of 600 households was invited to participate, and 1007 individuals from 405 households participated. 919 Provided serology data. A total of 6.0% of sample was made up of those aged 5-14 y.	NA	A total of 62% of the 88 participants who could not be assessed were children not assessed for technical reasons.	Positive findings on serology (IgG).	5-14 y; ≥15 y

Abbreviations: COVID-19, coronavirus disease 2019; ELISA, enzyme-linked immunosorbent assay; NA, not applicable; RT-PCR, reverse transcriptase–polymerase chain reaction; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.