Table 2. Main characteristics of included Systematic Reviews.

Study ID (n=20)	Fulfils systematic review methods	Research question (search date up to)	No. of included studies (No. of participants)	Main results	Key conclusions
Chen 2021	Yes	To estimate seroprevalence by different types of exposures, within each WHO region, we categorized all study participants into five groups: 1) close contacts, 2) high-risk healthcare workers, 3) low-risk healthcare workers, 4) general populations, and 5) poorly defined populations (Search from Dec 1, 2019, to Sep 25, 2020).	230 studies involving 1,445,028 participants were included in our meta-analysis after full-text scrutiny: Close contacts 16 studies 2901 positives out of 9,349 participants.	Estimated seroprevalence of all infections, 22.9% [95% CI, 11.1-34.7] compared to relatively low prevalence of SARS-CoV-2 specific antibodies among general populations, 6,5% (5.8-7.2%) The overall risk of bias was low.	There were a very limited number of high-quality studies of exposed populations, especially for healthcare workers and close contacts, and studies to address this knowledge gap are needed. Pooled estimates of SARS-CoV-2 seroprevalence based on currently available data demonstrate a higher infection risk among close contacts and healthcare workers lacking PPE.
Chu 2020	Yes	To investigate the effects of physical distance, face masks, and eye protection on virus transmission in healthcare and non-healthcare (e.g., community) settings (Searched up to March 26, 2020)	Identified 172 studies; 44 studies included in the meta-analysis which 7 were Covid-19.	A strong association was found of proximity of the exposed individual with the risk of infection (unadjusted n=10 736, RR 0·30, 95% CI 0·20 to 0·44; adjusted n=7782, aOR 0·18, 95% CI 0·09 to 0·38; absolute risk [AR] 12·8% with shorter distance vs 2·6% with further distance, risk difference. There were six studies on COVID-19, the association was seen irrespective of causative virus (p value for interaction=0·49). The risk of bias was generally low-to-moderate.	Physical distancing of at least 1m is strongly associated with protection, but distances of up to 2m might be more effective.
Fung 2020	Yes	To review and analyze available studies of the household SARs for SARS-CoV-2. Searched PubMed, bioRxiv, and medRxiv on 2 September 2020 for published and prepublished studies reporting empirical estimates of household SARs for SARS-CoV-2. Considered only English-language records posted on or after 1 January 2019.	22 papers met the eligibility criteria: 6 papers reported results of prospective studies and 16 reported retrospective studies. The number of household contacts evaluated per study ranged from 11 to 10592.	The 22 studies considered 20 291 household contacts, 3151 (15.5%) of whom tested positive for SARS-CoV-2. Household secondary attack rate estimates ranged from 3.9% in the Northern Territory, Australia to 36.4% in Shandong, China. The overall pooled random-effects estimate of SAR was 17.1% (95% confidence interval [CI], 13.7–21.2%), with significant heterogeneity (p<0.0001). The household secondary attack rate was highest for index cases aged 10–19 years (18.6%; 95% CI, 14.0–24.0%) and lowest for those younger than 9 (5.3%; 95% CI, 1.3–13.7%). Four of the studies were judged to be of high quality; 14 as moderate quality; and 4 as low quality. Between-study variation could not be explained by differences in study quality.	Secondary attack rates reported using a single follow-up test may be underestimated and testing household contacts of COVID-19 cases on multiple occasions may increase the yield for identifying secondary cases. There is a critical need for studies in Africa, South Asia, and Latin America to investigate whether there are setting- specific differences that influence the household SAR.
Goodwin 2021	Yes	What evidence is there for the transmission in indoor residential settings? What evidence is there for transmission in indoor workplace settings? What evidence is there for transmission in other indoor settings (social, community, leisure, religious, public transport)? Do particular activities convey greater risk (e.g. shouting, singing, eating together, sharing bedrooms)? What evidence is there for the appropriate length of distancing between people? Searches were conducted in May 2020 in PubMed, medRxiv, arXiv, Scopus, WHO COVID-19 database, Compendex & Inspec.	58 articles were included.	Pooled secondary attack rate within households was 11% (95%CI = 9, 13). There were insufficient data to evaluate the transmission risks associated with specific activities.	The overall quality of the evidence was low.
Irfan 2021	Yes	To assess transmission and risks for SARS-CoV-2 in children (by age- groups or grades) in community and educational-settings compared to adults. Searches conducted in PubMed, EMBASE, Cochrane Library, WHO COVID-19 Database, China National Knowledge Infrastructure (CNKI) Database, WanFang Database, Latin American and Caribbean Health Sciences Literature (LILACS), Google Scholar, and preprints from medRixv and bioRixv) covering a timeline from December 1, 2019, to April 1, 2021.	90 studies were included.	In educational-settings, children attending daycare/preschools (OR = 0.53, 95% CI = 0.38- 0.72) were observed to be at lower-risk when compared to adults, with odds of infection among primary (OR = 0.85, 95% CI = 0.55-1.31) and high-schoolers (OR = 1.30, 95% CI = 0.71- 2.38) comparable to adults. 28/29 prevalence studies were of good quality while one was of fair quality. 25/31 of contact- tracing studies were of good quality while six were of fair quality. 22/30 of studies conducted in educational settings were good quality while eight were of fair quality.	Children and adolescents had lower odds of infection in educational settings compared to community and household clusters.
Koh 2020	Yes	The secondary attack rate (SAR) in household and healthcare settings. Search between Jan 1 and July 25, 2020.	118 studies, 57 were included in the meta- analyses.	Pooled household secondary attack rate was 18.1% (95% CI: 15.7%, 20.6%) significant heterogeneity (P<0.001). No significant difference in secondary attack rates in terms of the definition of household close contacts, whether based on living in the same household (18.2%; 95% CI: 15.3%, 21.2%) or on relationships such as family and close relatives (17.8%; 95% CI: 13.8%, 21.8%) In three studies, the household secondary attack rates of symptomatic index cases (20.0%; 95% CI: 11.4%, 28.6%) was higher than asymptomatic ones (4.7%; 95% CI: 1.1%, 8.3%) Secondary attack rate from 14 studies showed close contacts adults were more likely to be infected compared to children (<18), relative risk 1.71 (95% CI: 1.35, 2.17). 43 high-quality studies were included for meta- analysis.	There was variation in the definition of household contacts; most included only those who resided with the index case, some studies expanded this to include others who spent at least a night in the same residence or a specified duration of at least 24 hours of living together, while others included family members or close relatives.
Li 2020	No (quality assessment not performed)	Carriage and transmission potential of SARS-CoV-2 in children in school and community settings (Search performed on 21 June 2020 with entry date limits from late 2019)	33 studies were included for this review. Four new studies on SARS-CoV-2 transmission in school settings were identified.	There is a lack of direct evidence on the dynamics of child transmission, however the evidence to date suggests that children are unlikely to be major transmitters of SARS-CoV-2.	The balance of evidence suggests that children play only a limited role in overall transmission, but it is noted that the relative contribution of children to SARS-CoV-2 transmission may change with reopening of society and schools.
Ludvigsson 2020	No (quality assessment not performed)	Are children the main drivers of the COVID-19 pandemic (Search to 11 May 2020)	47 full texts studied in detail.	This review showed that children constituted a small fraction of individuals with COVID-19.	Children are unlikely to be the main drivers of the pandemic. Data on viral loads were scarce but indicated that children may have lower levels than adults.
Madewell 2020	Yes	What is the household secondary attack rate for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)? ( Searched through Oct 19, 2020) single database assessed	54 studies with 77,758 participants were included.	Household secondary attack rate was 16.6%; restricted index cases to children (<18 years), lower SAR of 0.5%. SAR for household and family contacts was 3 times higher than for close contacts (4.8%; 95% CI, 3.4%-6.5%; P<0.001). Estimated mean household secondary attack rates from symptomatic index cases was significantly higher than from asymptomatic or presymptomatic index cases (18% vs 0.7%, P<0.001). Estimated mean household secondary attack rates to spouses (37.8%; 95% CI, 25.8%-50.5%) was higher than to other contacts (17.8%; 95% CI, 11.7%-24.8%). Significant heterogeneity was found among studies of spouses (I2 = 78.6%; P<0 .001) and other relationships (I2 = 83.5%; P<0.001). Contact frequency with index case associated with higher odds of infection. At least 5 contacts during 2 days before the index case was confirmed; at least 4 contacts and 1 to 3 contacts, or frequent contact within 1 meter. Secondary attack rates for households with 1 contact was significantly higher than households with at least 3 contacts (41.5% vs 22.8%, P<0.001) but not different than households with 2 contacts. There was significant heterogeneity in secondary attack rates between studies with 1 contact (I2 = 52.9%; P = .049), 2 contacts (I2 = 93.6%; P<0.001), or 3 or more contacts (I2 = 91.6%; P<0 .001). 16 of 54 studies (29.6%) were at high risk of bias, 27 (50.0%) were moderate, and 11 (20.4%) were low.	Secondary attack rates were higher in households from symptomatic index cases than asymptomatic index cases, to adult contacts than to child contacts, to spouses than to other family contacts, and in households with 1 contact than households with 3 or more contacts. Our study had several limitations. The most notable is the large amount of unexplained heterogeneity across studies. This is likely attributable to variability in study definitions of index cases and household contacts, frequency and type of testing, sociodemographic factors, household characteristics (e.g., density, air ventilation), and local policies (e.g., centralized isolation). The findings of this study suggest that households are and will continue to be important venues for transmission, even where community transmission is reduced.
Madewell 2021	No (quality assessed in previous review)	To further the understanding of SARS- CoV-2 transmission in the household. PubMed and reference lists of eligible articles were used to search for records published between October 20, 2020, and June 17, 2021.	A total of 87 studies representing 1 249 163 household contacts from 30 countries.	The estimated household secondary attack rate for all 87 studies was 18.9% (95% CI, 16.2%- 22.0%). Quality of included studies not reported.	Household remains an important site of SARS-CoV-2 transmission.
Qiu 2021	Yes	To critically appraise available data about secondary attack rates from people with asymptomatic, pre- symptomatic and symptomatic SARS- CoV-2 infection. Medline, EMBASE, China Academic Journals full-text database (CNKI), and pre-print servers were searched from 30 December 2019 to 3 July 2020.	80 studies were included.	Majority of studies identified index cases with a clear diagnosis, had an acceptable case definition and sufficiently followed up close contacts (for a minimum of 14 days). However, in some studies the definition of close contact and setting of transmission was not provided. The overall reporting quality was uncertain. Summary secondary attack rate estimate for asymptomatic cases was 1% (95% CI 0%–2%). The summary secondary attack rate estimate for presymptomatic index subjects was 7% (95% CI 3%–11%). The summary estimate of secondary attack rate from symptomatic index subjects was 6% (95% CI 5%–8%).	Asymptomatic patients can transmit SARS-CoV-2 to others, but such individuals are responsible for fewer secondary infections than people with symptoms.
Shi 2022	Yes	To examine the transmissibility and pathogenicity of COVID-19 reflected by the secondary infection rate (SIR), secondary attack rate (SAR), and symptomatic infection ratio. Searches were conducted in Web of Science and PubMed, and Chinese databases, including China National Knowledge Infrastructure, WANFANG Database, and the VIP Database for Chinese Technical Periodicals. 17 August 2020	A total of 105 studies were identified, with 35042 infected cases and 897912 close contacts.	28 studies were of high quality, 66 studies were of moderate quality, and 11 were of low quality. The secondary attack rate was 6.6% (95% CI, 5.7%−7.5%). Household contact had significantly higher secondary attack rate (19.6%, 95% CI [15.4–24.2%]) than community contact (SAR, 8.1%, 95% CI [5.2–11.5%]; P=0.013) and medical contact (SAR, 3.8%, 95% CI [0.9–8.4%]; P<0.001).	There is a higher risk of infection among household contacts.
Silverberg 2022	Yes	To identify the role of children in SARS- CoV-2 transmission to other children and adults. MEDLINE, EMBASE, CINAHL, Cochrane Central Register of Controlled Trials, and Web of Science were electronically searched for articles published before March 31, 2021.	40 articles were included. 357 paediatric index cases.	The overall SAR was 8.4% among known contacts (5.7% in children and 26.4% in adults). Children were significantly less likely to be infected than adults: OR 0.21 (95% CI 0.05-0.91), with no heterogeneity (I2=0%) Ten were deemed to be of good quality and have low risk of bias, while 22 were of fair quality and 8 were of poor quality.	Children transmit COVID-19 at a lower rate to children than to adults. Household adults are at highest risk of transmission from an infected child.
Thompson 2021	Yes	To estimate SAR of SARS-CoV-2 in households, schools, workplaces, healthcare facilities, and social settings. Searches were conducted in MEDLINE, Embase, MedRxiv, BioRxiv, arXiv, and Wellcome Open Research with no language restrictions up to July 6, 2020.	45 studies were included for meta-analysis.	Household SAR was 21.1% (95%CI: 17.4–24.8). The SAR increased with longer durations of exposure (14.2% [95% CI: 5.8–22.5] with ≤5 days of exposure to an index case vs 34.9% [95%CI 16.3–53.6] with >5 days of exposure; P=0.05. SARs were significantly higher for presymptomatic and symptomatic index cases, estimated at 9.3% (95% CI: 4.5–14.0, P=0.01) and 13.6% (95%CI 9.7–17.5, P<0.001), respectively. Articles that met the inclusion criteria for meta- analysis all had high quality scores.	Exposure in settings with familiar contacts increases SARS- CoV-2 transmission potential.
Tian 2020	Yes	Searched published literatures and preprints in international databases of PubMed and medRxiv, and in five major Chinese databases as of 20 April 2020	18 studies were included for meta-analysis. A total of 32,149 close contacts were documented.	The pooled SAR was 0.07 (95%CI 0.03-0.12). Household setting and social gatherings were associated with significantly elevated SARs (P<0.01). 17 studies were high quality, and one was moderate quality using the AHRQ criteria.	The transmission risk of SARS-CoV-2 is much higher in households than in other scenarios.
Viner 2021	Yes	To assess child and adolescent susceptibility to SARS-CoV-2 compared with adults. Searched 2 electronic databases, PubMed and the medical preprint server medRxiv, on May 16, 2020, and updated this on July 28, 2020	32 studies comprising 41 640 children and adolescents and 268 945 adults met inclusion criteria.	The pooled odds ratio of being an infected contact in children compared with adults was 0.56 (95% CI, 0.37-0.85), with substantial heterogeneity (I2=94.6%). Two studies were high quality, 22 were medium quality, 7 were low quality, and 1 was uncertain quality.	Children have a lower susceptibility to SARS-CoV-2 infection compared with adults
Viner 2022	Yes	Research questions: (a) To what extent do CYP under 20 years of age transmit SARS-CoV- 2 to other CYP and to adults in household and child-specific (e.g. educational) settings?; (b) how does transmission differ between household and educational settings?; and (c) is community infection incidence associated with prevalence of or transmission of infection within educational settings? Searched four electronic databases (PubMed; medRxiv; COVID-19 Living Evidence database; Europe PMC) to 28 July 2021.	37 studies were included.	The pooled estimates of SAR were 7.6% (3.6, 15.9) for household studies, significantly higher than the pooled estimate for school studies of 0.7% (0.2, 2.7), P=0.002)). Across all studies, pooled risk of transmission was lower from child index cases than adults (OR 0.49 (0.25, 0.98). 24 studies had high quality, and 13 were medium quality.	SAR were markedly lower in school compared with household settings, suggesting that household transmission is more important than school transmission in this pandemic.
Xu 2020	Yes	Evidence for transmission of COVID- 19 by children in schools ( search in MEDLINE up to 14 September 2020. Further hand-searched reference lists of the retrieved eligible publications to identify additional relevant studies). Included children (defined as ≤18 years old) who were attending school, and their close contacts (family and household members, teachers, school support staff) during the COVID-19 pandemic	11 studies were included: 5 cohort studies and 6 cross- sectional studies.	Overall infection attack rate (IAR) in cohort studies: 0.08%, 95% CI 0.00%-0.86%. IARs for students and school staff were 0.15% (95% CI 0.00%-0.93%) and 0.70% (95% CI = 0.00%-3.56%) respectively (p<0.01). Six cross-sectional studies reported 639 SARS-CoV-2 positive cases in 6682 study participants tested [overall SARS-CoV-2 positivity rate: 8.00% (95% CI = 2.17%-16.95%). SARS-CoV-2 positivity rate was estimated to be 8.74% (95% CI = 2.34%-18.53%) among students, compared to 13.68% (95% CI = 1.68%- 33.89%) among school staff (p<0.01). Overall study quality was judged to be poor with risk of performance and attrition bias.	There is limited high-quality evidence to quantify the extent of SARS-CoV-2 transmission in schools or to compare it to community transmission. Emerging evidence suggests lower IAR and SARS-CoV-2 positivity rate in students compared to school staff.
Yanes-Lane 2020	Yes	Proportion of asymptomatic infection among coronavirus disease 2019 (COVID-19) positive persons and their transmission potential. (Search up to up to 22 June 2020)	28 studies were included.	Asymptomatic COVID-19 infection at time of testing ranged from 20% – 75%; among three studies in contacts it was 8.2% to 50%. Asymptomatic infection in obstetric patients pooled proportion was 95% (95% CI, 45% to 100%) of which 59% (49% to 68%) remained asymptomatic through follow-up; Among nursing home residents, the proportion of asymptomatic was 54% (42% to 65%) of which 28% (13% to 50%) remained asymptomatic through follow-up. The overall quality of included studies was moderate-to-high.	The proportion of asymptomatic infection among COVID-19 positive persons appears high and transmission potential seems substantial.
Zhu 2021	Meta- analysis: Quality assessment not reported	Role of children in SARS-CoV-2 in household transmission clusters ( Search between Dec 2019 & Aug 2020).	57 articles with 213 clusters were included.	8 (3.8%) transmission clusters were identified as having a paediatric index case. Asymptomatic index cases were associated with lower secondary attack rates in contacts than symptomatic index cases [RR] 0.17 (95% CI,0.09- 0.29). SAR in paediatric household contacts was lower than in adult household contacts (RR, 0.62; 95% CI, 0.42-0.91).	The data suggest that should children become infected at school during this period, they are unlikely to spread SARS- CoV-2 to their co-habiting family members.