Table 3. Relevant evidence base on transmission of SARS-CoV-2 by children.

Study (First author, reference number, country and time of data collection)	Study design, Study Population Sample size	Main findings*
Zhu Y. (25), China, Singapore, South Korea, Japan and Iran; December 2019 / March 2020 [house-style]	Summary of household cluster studies comparing proportion pediatric index cases and secondary infections during SARS-CoV-2 epidemic versus H5N1 epidemic; 87 households	In 3/31 (9.7%) SARS-CoV-2 household transmission clusters the index case was a child versus in 30/56 (54%) H5N1 clusters
Posfay-Barbe K. (26), Switzerland; March 2020 / April 2020	Household cluster study of first pediatric (<16 yo) sars-cov-2 pcr+ cases; 40 positive children and 111 household contacts	In 31/39 (71%) households adults assigned as index case versus 3/39 (8%) households with child symptoms prior to other household members
Dattner I. (e24), Israel, start detection of cases—May 2020; (Somekh E.(27) publication of initial families)	Household cluster study of households with at least 1 PCR+ case, all family members tested; 637 households, n=3353; children <21 yo n=1544; initial 13 families: 36 adults (>18 yo); 58 children	– 58 households with probability=1 that index case is adult; probability=0 in 34 households – Initial 13 families: one 14 yo infector. – Relative infectivity of children versus adults (modeled) was 0.85 (95% 0.65–1.10)
NCIRS (28), Australia: 5 March 2020–3 April 2020	School Outbreak investigation in 10 high schools and 5 primary schools of close contacts defined as “face to face contact for ≥15 minutes or in the same room for two hours with a case while infectious”; contacts: students n=735, staff n=128 exposure to 9 infected students and 9 infected staff	– 10 high-schools: 8 PCR+ students, 4 PCR+ staff, 598 students, 97 staff contacts; 235 contacts PCR test: all neg; 75 serology tested: 1 positive student – 5 primary high schools, 1 PCR+ student, 5 PCR+ staff, close contacts 137 students, 31 staff. PCR test 53: 1 PCR+; 21 serology tested: same student sero-positive
Fontanet (29), France: 13 January 2020–27 March 2020	School Outbreak investigation; serology testing (IgG); infection attack rate; 15–17 yo students (n=240), teachers (n=53), and students’ parents (n=211) and siblings (n=127) – total: n=661	Sero-positivity: 38.3% 15–17 yo pupils; 2.7% of siblings <14 yo, 10.2% siblings (all ages) and 11.4% parents, teachers 43.4%, 59.3% school staff
Haevey (30), Ireland; prior to 12 March 2020	School Outbreak investigation; n=1155 contacts (includes school contacts: children n=924, adults 101) of 6 PCR+ cases; contacts clinical follow up, symptomatic contacts PCR tested	– SARS-CoV-2 PCR+ cases: 1 primary school (10–15 yo), 2 high-school (10–15 yo) PCR+ cases; 3 infected adults; – Follow-up testing did not show school-related secondary cases – 2 secondary cases through adult-to-adult contact outside school environment
Hildenwall (e25), Sweden; 13 March 2020–14 May 2020	Cases during school opening, children 0–18 yo n=63 cases	– 63 admitted cases 0–17 yo, 30 non-incidental cases; hospitalization rate for COVID-19 cases of 9/100 000; – proportion of all SARS-CoV-2-positive admitted children 16–18 yo (10/63, 16%), for whom schools have been operating on distance, children 1–5 yo (11/63, 17%)
Stringhini (31), Switzerland (Geneva); 6 April 2020–9 May 2020	Sero-prevalence study population samples (SEROCoV-POP), ELISA adjusted for test performance, weekly during 5 weeks, all ages (n=2766, 1339 households), children (n=455)	– In 5–19 yo: 6% sero-positivity vs in 20–49 yo: 8.5%; – relative risk (95%CI) sero-positivity compared to 20–49 yo: 0.32 (0.11–0.63) 5–9 yo, 0.86 (0.57–1.22) in 10–19 yo
Cohen (33), France; 14 April 2020–12 May 2020	Cross-sectional (population-enrolled by ambulatory pediatricians) survey using PCR and sero-prevalence, during confinement, 0–15 yo (n=605)	PCR+ 11 (1.8%), sero-positive 65 (10.7%); positivity independent of number of children in the household
Estudio ENE second round (32), Spain;18 May 2020–1 June 2020	Sero-prevalence study, population wide, IgG rapid test positives; 0–19 yo n=11 730 (<1yo: 263, 1–4 yo: 1679, 5–9 yo: 2896, 10–14 yo: 3549, 15–19 yo: 3343)	5–9 yo: 2.9% (2.2–4.0) sero-positivity (95%CI) vs 5.2% (4.9–5.5) overall; (<1 yo: 2.2% [0.7–6.8], 1–4 yo:2.4% [1.5–3.8], 5–9 yo:2.9% [2.2–4.0], 10–14 yo: 3.8% [3.0–4.8], 15–19 yo: 3.8% [3.0–4.8])
Havers (e27), USA (Connecticut, South Florida, Missouri, NYC, Utah, Washington state); 23 March 2020–3 May 2020	Sero-prevalence study, convenience sample of residual clinical blood samples commercial laboratories; ELISA (Spike protein, screening plus confirmatory essay) adjusted for sensitivity and specificity; children 0–18 yo n= 1001, limited number <5yo, all ages n=11 933,	Sero-positivity (95%CI) for 0–18 yo vs all-ages, age-standardized: WA 0.66 (0.0, 2.52) vs 1.13 (0.70, 1.94) NYC 2.74 (0.9, 5.03) vs 6.93 (5.02, 8.92) FL 2.41 (0.00, 7.79) vs 1.85 (1.00, 3.23) MI 1.36 (0.00, 4.14) vs 2.65 (1.65, 3.86) UT (insufficient pediatric data) CT 0.81 (0.00, 2.89) vs 4.94 (3.61, 6.52)
L‘Huillier (34), Switzerland; early pandemic	Clinical laboratory, viral load study, RT-PCR and cell culture, n=23 (0–16 yo) symptomatic children	– Cultivable SARS-CoV-2 can be present in URT naso-pharyngeal specimens from symptomatic children, isolation success 12/23 (52%) children; – Median viral load at time of diagnosis was 3.0x106 copies/ml (mean 4.4x108, IQR 6.9x103–4.4x108).
Jones (35), Germany; 26 January 2020–May 2020	Clinical laboratory, viral load study (VL 250 000 copies threshold for isolation, infectious virus in cell culture at 5% probability=infectiousness), total n=3303 PCR+, ages 1–19 yo n=154	Infectiousness: 29.0% of 38 patients 0–6 yo, 37.3% of 0–19 yo (n=150) versus 51.4% among 3153 adults
Stage (e28), data from Germany, Denmark, Norway, Sweden; March 2020–June 2020	Time-series, comparative analysis of school-closures and school-reopening using case studies; effect on growth rates in daily hospitalizations or confirmed cases, accounting for – different age cohorts of students – timing of closure and reopening – background incidence	– No increased transmission in low community transmission settings after return of all students – No increased transmission in higher community transmission settings after partial return of younger year students – Increased transmission in students but not in staff on return more older students – School closures partially but non-differentially responsible for reduction growth rate
Ferguson (37) June 2020	Mathematical modeling study; epidemiological modeling of the impact of non-pharmaceutical interventions, including school closure – Assumption of children attack rates: equal as in influenza; children transmit as much as adults	– School closure is predicted to be insufficient to mitigate an epidemic in isolation – Combining social distancing of the entire population, case isolation, household quarantine and school and university closure predicted largest impact – Limited impact of school closures at the peak, prolonged closure necessary to maintain controlled transmission
Matrajt (36), modelling population Seattle metropolitan area; June 2020	Mathematical modeling study; Assumptions: children and adults equally infectious, children group with highest number of contacts; children <19yo	– Targeting to all age groups, delayed the epidemic the longest, >50 days – Targeting adults >60 yo and children resulted in 10 500 (45%) fewer cases than baseline at the epidemic peak; targeting adults-only: 11 000 (47%) fewer cases for 25% reduction in contacts in adults <60 yo and 21 000 (91%) fewer cases for 75% adult contact reduction
Davies (38), Data from: Canada, China, Italy, Japan, Singapore – model for Milan, Birmingham, Bulawayo; June 2020	Mathematical modeling study; population-wide estimates; assumptions: relative susceptibility toinfection in children 0–9 yo: 0.4; infectivity in children based on clinical fraction 0.29 in 0–9 yo. Infectivity dependent on clinical fraction and infectiousness 0 to 100% of clinical cases	– Interventions aimed at children had/showed relatively small impact on reducing transmission, particularly if the transmissibility of subclinical infections is low – Simulation 3 months of school closures: school closure decreased peak incidence 10–19% vs 17–35% for influenza

ELISA, Enzyme-Linked Immunosorbent Assay; CT = “Connecticut”; FL = “Florida”; IQR, interquartile range; MI = “Missouri”; NYC = “New York City”; PCR = “polymerase chain reaction for Sars-CoV-2”; URT = “upper respiratory tract”; UT =”Utah”; WA = “Washington State”; yo = “years old”; 95%CI = “95% Confidence Interval”

*A detailed critical review of the available evidence is beyond the scope of this table. Every study needs to be assessed for selection bias, information bias, confounding, survivor bias, interpretation of diagnosis assessment, the study population and context (timing in the epidemic, social context etc) where this was performed and the statistical soundness of the analysis. The main findings are to be interpreted by the reader taking at minimum these aspects into account.