The role of children in household transmission of COVID-19: a systematic review and meta-analysis.

OBJECTIVES: To explore household transmissibility of SARS-CoV-2 in children in new-variants dominating periods.
METHODS: Through retrieval in PubMed and Embase, studies were included in two parts: meta-analysis of the household secondary attack rate (SAR) and case analysis of household pediatric infections.
RESULTS: A total of 95 articles were included: 48 for meta-analysis and 47 for case analysis. Pediatric COVID-19 only comprised a minority of the household transmission. The total pooled household SAR of child index cases and contacts were 0.20 (95% confidence interval [CI]: 0.15-0.26) and 0.24 (95% CI: 0.18-0.30). Lower household transmissibility was reported in both child index cases and contacts than in adults (relative risk [RR] = 0.64, 95% CI: 0.50-0.81; RR = 0.74, 95% CI: 0.64-0.85). Younger children were as susceptible as the older children (RR = 0.89, 95% CI: 0.72-1.10). Through subgroup analyses of different variants and periods, increased household SAR was observed in children (Wild: 0.20; Alpha: 0.42; Delta: 0.35; Omicron: 0.56), and no significant difference was found in household SAR between children and adults when new variants dominated.
CONCLUSION: Although children were found not to be dominant in the household transmission, their transmissibility of SARS-CoV-2 appeared to be on the rise as new variants emerged.

Introduction

As of April 29, 2022, there have been 510.2 million confirmed COVID-19 cases and 6.2 million confirmed deaths worldwide, and individuals around the world are still experiencing the aftermath of the fourth wave of the pandemic, which was caused by the Omicron variant of SARS-CoV-2 (WHO COVID-19 Dashboard Data, 2022).

For outbreak control, breaking the chain of virus transmission is generally considered to be one of the most effective strategies besides vaccination. Previous studies have suggested that the household is potentially the highest-risk exposure setting of SARS-CoV-2 transmission, which may have led to a steep escalation of COVID-19 cases even after the policy of national lockdowns and extreme social distancing norms in many countries (Chakrabarti et al., 2020; Coccia, 2020; Lewis et al., 2021). Children often play an important role in the transmission of some respiratory infectious diseases, such as influenza and measles (García‐Salido, 2020; Viner et al., 2020; Yang, 2020). However, for SARS-CoV-2, it remains controversial (García‐Salido, 2020; Goldstein et al., 2021; Lau et al., 2020; Lee and Raszka, 2020). Pediatric infections only comprise a small proportion of the total reported cases and children are usually reported with a lower infection rate and a milder clinical course compared with adult cases (Dong et al., 2020; Hoang et al., 2020; Irfan et al., 2021a; Ye et al., 2020). However, children may represent an essential chain of viral transmission and be responsible for the continuous spread of the virus on account of children frequently being asymptomatic carriers (de Souza et al., 2020; Irfan et al., 2021b). With the emergence of some new virus variants, such as Delta and Omicron, increased transmissibility of SARS-CoV-2 in children has been reported by many studies (Chun et al., 2022; Cloete et al., 2022; Elliott et al., 2022; Marks et al., 2022; Thelwall et al., 2022). What is worse is that although vaccinations for adults are ongoing, there is still a vacuum in children, especially for those younger than 12 years (Walter et al., 2022), which also may be an important reason for the viral transmission (Li et al., 2022).

Because an understanding of the role of children in the household transmission of SARS-CoV-2 is still evolving, further analysis is necessary. This study aimed to (1) assess the prevalence of pediatric COVID-19 in family clusters, (2) estimate the household secondary attack rate (SAR) of children in different periods and variants, and (3) compare the transmissibility of SARS-CoV-2 in different age groups and explore its potential determinants.

Methods

This systematic review and meta-analysis were conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, and the protocol was registered on PROSPERO (CRD42022313960).

Definition

A household transmission cluster was defined as a group of ≥2 confirmed COVID-19 cases in cohabiting individuals where the diagnosis of cases occurred within 2 weeks of each other. The index case, the primary case, was defined as the first person in the household to be infected with SARS-CoV-2. Household contacts were defined as family members or close relatives who had unprotected contact with the index case but did not necessarily live together. The transmissibility of SARS-CoV-2 was empirically estimated by the SAR. The household SAR was defined as the number of household secondary cases divided by the total household contacts. Children were individuals aged <18 years. Notably, for studies dividing the age groups by 10 years, individuals aged 10–19 years were included in the child group.

Search strategy and eligibility criteria

A systematic retrieval was performed on two databases (PubMed and Embase) from inception to April 20, 2022, using the key search terms: COVID-19, SARS-CoV-2, family characteristics, household transmission, and so on (details in Table S1), with no restriction on the language, date, study type, or place of publication. Nonprimary documents and modeling studies were excluded.

Depending on the study type and provided data, studies were included in two parts: case analyses of household pediatric infections and meta-analysis on the household SAR. Case analyses mainly included case reports focusing on individual household transmission of SARS-CoV-2. The personal information of index cases, household contacts, family relationships, and the disease progression of COVID-19 cases must be provided. Although the SAR meta-analysis mainly included descriptive studies that had reported the household SARS-CoV-2 SAR in different age groups, at least two of the following were required: household contacts, household secondary cases, and SAR. Studies with insufficient data or possible duplicate cases were excluded.

Data extraction and quality assessment

Two authors (Tian and Zhang) independently extracted the following information from each of the included study: author, country, study type, study period, case definitions, testing protocol, contact tracing methods, demographic characteristic, COVID-19 data (exposures, index cases, household contacts, secondary infection cases, SAR), potential factors, and so on. Disagreements were resolved through consultation with the third author (Chen). To critically appraise the methodologic quality of included studies, the JBI critical appraisal checklist was applied (JBI, 2020). Each included study was scored independently by two authors (Tian and Zhang) and was given an average point. Studies were ranked as high quality if they were scored ≥10, medium if they were scored 7–9, and low if they were scored <7.

Data analysis

All analyses were performed using R 4.1.2 software. The SAR and its relative risk (RR) were calculated for each study. SARs were pooled with a random intercept logistic regression model after a Freeman-Tukey double arcsine transformation, and RRs were pooled using a random-effects model with Der Simonian and Laird weights. The within-study variation was estimated with the 95% confidence interval (CI), and the Higgin and Thompsons I2 was used to assess heterogeneity between studies. Subgroup analyses were conducted to explore the source of heterogeneity. Publication bias was detected using the funnel plot and Egger test. P < 0.05 was considered statistically significant in all tests.

Results

As shown in the flow diagram in Figure S1, a total of 1632 records were identified through the data search and 236 articles were retrieved for full-text assessment. Finally, 95 articles were included in our analysis: 48 articles for household SAR meta-analysis and 47 articles for case analysis. Studies included in the SAR meta-analysis are listed in Table 1 , of 48 studies, 26 were of high quality and 22 were of medium quality according to the quality assessment in Table S2, and the full details of family clusters included in case analyses are shown in Table S3. All included studies reported household COVID-19 from 18 countries and regions with a total of 1,153,693 participants (834,613 adults and 319,080 children).

Table 1

Studies included in meta-analysis of household SAR.

Author (year)	Country	Study type		Cluster size	Public lockdown	Diagnostic method	Follow-up (days)	Quality
Afonso et al. (2022)	Brazil	Cross-sectional and analytical study		NA	Yes	RT-PCR	14	Medium
Baker et al. (2022)	United States	Retrospective study		183	NA	RT-PCR	NA	High
Bhatt et al. (2022)	Canada	Prospective study		180	NA	RT-PCR	14	High
Bi et al. (2021)	Switzerland	Cross-sectional population serosurvey		2267	Yes	Serological test	NA	High
Bi et al. (2020)	China	Retrospective cohort study		NA	Yes	RT-PCR	14	High
Calvani et al. (2021)	Italy	Case-control study		NA	NA	RT-PCR	NA	Medium
Cerami et al. (2021)	United States	Prospective study		100	NA	RT-PCR	28	High
Chaw et al. (2020)	Malaysia	Retrospective study		28	NA	RT-PCR	14	Medium
de Gier et al. (2021)	The Netherlands	Retrospective study		NA	NA	RT-PCR	10	Medium
Donnelly et al. (2022)	United States	Prospective study		127	NA	RT-PCR	14	High
Dupraz et al. (2021)	Switzerland	Cross-sectional epidemiological study		NA	Yes	Serological test	14	Medium
Galow et al. (2021)	Germany	Seroprevalence study		106	NA	Serological test	NA	Medium
Harris et al. (2021)	England	Retrospective study		NA	NA	RT-PCR	14	High
Hu et al. (2021)	China	Retrospective cohort study		NA	Yes	RT-PCR	14	High
Hua et al. (2020)	China	Retrospective cohort, multicenter study		314	Yes	RT-PCR	NA	Medium
Jalali et al. (2022)	Norway	Cohort study		NA	NA	RT-PCR	10	High
Jing et al. (2020)	China	Retrospective cohort study		195	Yes	RT-PCR	14	High
Kim et al. (2021)	South Korea	Retrospective observational study		NA	NA	RT-PCR	NA	Medium
Koureas et al. (2021)	Greece	Retrospective cohort study		40	Yes	RT-PCR	NA	Medium
Kuba et al. (2021)	Japan	Cohort study		NA	Yes	RT-PCR	14	Medium
Lewis et al. (2021)	United States			58	Yes	RT-PCR	14	High
Li et al. (2021)	China	Retrospective cohort study		24985	Yes	RT-PCR	≥22	High
Li et al. (2020)	China	Retrospective study		105	NA	RT-PCR	14	Medium
Liu et al. (2021)	United States	Prospective study		15	NA	RT-PCR	14	High
Lopez Bernal et al. (2022)	England	Prospective case-ascertained study		329	NA	RT-PCR	14	High
Lyngse et al. (2022)	Denmark	Retrospective study		24693	NA	RT-PCR	14	High
McLean et al. (2022)	United States	Prospective case-ascertained study		302	NA	RT-PCR	14	High
Metlay et al. (2021)	United States	Retrospective cohort study		NA	NA	RT-PCR	NA	Medium
Miller et al. (2021)	England	Prospective cohort study		NA	NA	RT-PCR	NA	Medium
Miyahara et al. (2021)	Japan	Cohort study		87	Yes	RT-PCR	14	Medium
Musa et al. (2021)	Bosnia and Herzegovina	Prospective observational study		360	NA	RT-PCR	28	High
Ng et al., 2022a	Malaysia	Retrospective observational study		185	Yes	RT-PCR	14	Medium
Ng et al., 2022b	Singapore	Retrospective cohort study		NA	Yes	RT-PCR	14	High
Ogata et al. (2021)	Japan	Cross-sectional study		183	Yes	RT-PCR	NA	Medium
Ogata et al. (2022)	Japan	Observational study		580	NA	RT-PCR	NA	High
Park et al. (2020)	South Korea	Cohort study		NA	NA	RT-PCR	14	Medium
Reukers et al. (2022)	The Netherlands	Prospective cohort study		55	NA	RT-PCR	NA	High
Rosenberg et al. (2020)	United States	Retrospective study		155	Yes	RT-PCR	NA	High
Song et al. (2022)	South Korea	Prospective study		25	NA	NA	NA	High
Soriano-Arandes et al. (2021)	Spain	Prospective, observational study		1108	Yes	RT-PCR	NA	Medium
Stich et al. (2021)	Germany	Multicenter, cross-sectional study		405	NA	Serological test	NA	High
Tanaka et al. (2021)	Japan	Cross-sectional study		NA	NA	RT-PCR	14	Medium
Waltenburg et al. (2022)	United States	Prospective study		127	NA	RT-PCR	14	High
Wang et al. (2020a)	China	Retrospective cohort study		124	NA	RT-PCR	14	High
Wang et al., (2020b)	China	Retrospective case series		85	Yes	RT-PCR	14	High
Wu et al. (2020)	China	Prospective observational study		35	NA	RT-PCR	NA	Medium
Yousaf et al. (2021)	United States	Prospective cohort study		NA	NA	RT-PCR	14	High
Yung et al. (2020)	Singapore	Prospective study		137	NA	RT-PCR	14	Medium

NA, not applicable; RT-PCR, reverse transcription polymerase chain reaction; SAR, secondary attack rate.

Case analyses of household pediatric COVID-19

In the case analysis of pediatric COVID-19, 47 articles were included, identifying 78 household transmission clusters. As shown in Table 2 , only 10.3% (8/78) familial clusters were identified with a pediatric index case. These pediatric index cases only led to 7.7% (16/207) of all secondary cases compared with the 92.3% of secondary cases caused by the adult index cases. Child contacts were identified as 29.8% (84/282) of all household contacts and reported in 60.3% (47/78) familial clusters. The child secondary infections only accounted for 30% (62/207) of all secondary infections compared with the 70% as adults.

Table 2

Case analyses of household pediatric COVID-19 infections.

Characteristics	Cluster (n = 78), %	Secondary cases (n = 207), %
Child as the index case	8 (10.3)	16 (7.7)
Adult as the index case	70 (89.7)	191 (92.3)
Child as the contacts	47 (60.3)	62 (30.0)
Adult as the contacts	77 (98.7)	145 (70.0)

COVID-19, coronavirus disease.

Meta-analyses on household SAR of SARS-COV-2

Household SAR of child contacts

Secondary infections of the pediatric household contacts were identified in 41 studies, and the pooled SAR was 0.24 (95% CI: 0.18–0.30, I2 = 100%) (Figure 1 ). Publication bias was reported upon examination of a funnel plot (Egger test, P = 0.021) (Figure S2).

Figure 1

Pooled household SAR of child contacts. CI, confidence interval; SAR, secondary attack rate.

Subgroup analyses on household SAR of child contacts were performed on research periods and SARS-CoV-2 variants, as provided in Table 3 . In different research periods, 31 studies were carried out between 2019 and February 2021, and the SAR was estimated at 0.18 (95% CI: 0.13–0.25, I2 = 99%). A total of 9 studies were conducted between February and November 2021, and the SAR was 0.39 (95% CI: 0.30–0.48, I2 = 97%). The SAR of two studies between November 2021 and 2022 was 0.51 (95% CI: 0.47–0.54, I2 = 0%). Significant difference in SAR was reported in different groups of research period (P < 0.01). For different SARS-CoV-2 variants, the SAR of Wild type in 33 included studies was 0.20 (95% CI: 0.14–0.26, I2 = 99%). The SAR of the Alpha variant in the three included studies was 0.42 (95% CI: 0.23–0.62, I2 = 94%). The Delta variant was investigated in five studies, and the SAR was 0.35 (95% CI: 0.25–0.45, I2 = 98%). The SAR of the Omicron variant in two studies was 0.56 (95% CI: 0.51–0.61, I2 = 20%). A significant difference in SAR was also reported among different variants (P < 0.01).

Table 3

Subgroup analyses on household SAR of child contacts.

Subgroups	No. of studies	SAR (95% CI)	I2	P-value
Research period				<0.01
2019-Feb, 2021	31	0.18 (0.13–0.25)	99%
Feb-Nov, 2021	9	0.39 (0.30–0.48)	97%
Nov, 2021-2022	2	0.51 (0.47–0.54)	0%
SARS-CoV-2 variant				<0.01
Wild type	33	0.20 (0.14–0.26)	99%
Alpha	3	0.42 (0.23–0.62)	94%
Delta	5	0.35 (0.25–0.45)	98%
Omicron	2	0.56 (0.51–0.61)	20%

CI, confidence interval; SAR, secondary attack rate; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

In the analyses on household SAR of child contacts in different age groups, children younger than 10 years were found to be less susceptible than children older than 10 years (RR = 0.74, 95% CI: 0.56–0.97, I2 = 0%). However, no significant difference was shown between children younger and older than 12 years (RR = 1.12, 95% CI: 0.90–1.39, I2 = 77%). In the combined analysis on the previous two cases, the younger child contacts were not significantly associated with a lower SAR than the older ones (RR = 1.01, 95% CI: 0.84–1.21, I2 = 66%) (Figure 2 ).

Figure 2

Subgroup analyses on household SAR of child contacts in different age groups. CI, confidence interval; RR, risk ratio; SAR, secondary attack rate.

Household SAR of adult contacts

The SAR of adult household contacts was estimated at 0.32 (95% CI: 0.27–0.37, I2 = 99%) on the basis of 41 included studies (Figure S3). Publication bias was also reported in the funnel plot of Figure S4 (Egger test, P < 0.01). In the analysis on adult household contacts of different age groups, the old adults were significantly associated with a higher SAR than young adults (>60 vs <60 years: RR = 1.45, 95% CI: 1.24–1.70, I2 = 52%; >65 vs <65 years: RR = 1.24, 95% CI: 1.02–1.50, I2 = 55%). The same trend was also found in the combined analysis (the old adults vs the young adults: RR = 1.35, 95% CI: 1.19–1.54, I2 = 77%) (Figure S5).

Household SAR comparison between child and adult contacts

In the household SAR comparison between child and adult contacts in 37 studies, children were demonstrated to be less likely to be infected with SARS-COV-2 than adults when exposed to household index cases (RR = 0.74, 95% CI: 0.64–0.85, I2 = 97%) (Figure 3 ). No obvious publication bias was found in the funnel plot of Figure S6 (Egger test, P = 0.31).

Figure 3

Household SAR comparison between child and adult contacts. CI, confidence interval; RR, risk ratio; SAR, secondary attack rate.

Subgroup analyses of the comparison were performed on research periods and SARS-CoV-2 variants, as detailed in Table 4 . In different research periods, 27 studies were carried out between 2019 and February 2021, in which lower transmissibility was reported in child contacts than adult contacts (RR = 0.62, 95% CI: 0.52–0.75, I2 = 95%). For nine studies between February and November 2021 and two studies between November 2021 and 2022, no significant difference in SAR was found between child and adult contacts (RR = 0.98, 95% CI: 0.86–1.12, I2 = 80%; RR = 1.09, 95% CI: 0.89–1.34, I2 = 73%). A significant difference in RR was reported in different groups of research period (P < 0.01). For different SARS-CoV-2 variants, children were significantly associated with a lower SAR than adult contacts in 29 studies of the Wild type variant (RR = 0.65, 95% CI: 0.55–0.77, I2 = 95%). However, no significant difference in SAR was observed between child and adult contacts in studies of other variants (Alpha: RR = 1.04, 95% CI: 0.76–1.42, I2 = 76%; Delta: RR = 0.99, 95% CI: 0.82–1.19, I2 = 88%; Omicron: RR = 1.09, 95% CI: 0.88–1.35, I2 = 74%). Significant difference in RR was also reported in different variants (P < 0.01).

Table 4

Subgroup analyses of household SAR comparison between child and adult contacts.

Subgroups	No. of studies	RR (95% CI)	I2	P-value
Research period				<0.01
2019–June, 2020	27	0.62 (0.52–0.75)	95%	<0.01
February–November, 2021	9	0.98 (0.86–1.12)	80%	>0.05
November, 2021–2022	2	1.09 (0.89–1.34)	73%	>0.05
SARS-CoV-2 variant				<0.01
Wild type	29	0.65 (0.55–0.77)	95%	<0.01
Alpha	3	1.04 (0.76–1.42)	76%	>0.05
Delta	5	0.99 (0.82–1.19)	88%	>0.05
Omicron	2	1.09 (0.88–1.35)	74%	>0.05

CI, confidence interval; RR, relative risk; SAR, secondary attack rate; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Household SAR of child and adult index cases

A total of 18 studies reported the respective SAR of child and adult index cases in familial clusters. The estimated SAR of the child index case was 0.20 (95% CI: 0.15–0.26, I2 = 100%). For the adult index cases, it was 0.36 (95% CI: 0.27–0.46, I2 = 100%). Compared with the adult index cases, the child index cases were significantly associated with a lower possibility to transmit SARS-CoV-2 to their family members (RR = 0.64, 95% CI: 0.50–0.81, I2 = 96%) (Figure 4 ).

Figure 4

Comparison on household SAR between child and adult index cases. CI, confidence interval; RR, risk ratio; SAR, secondary attack rate.

Potential determinants of the household SAR

Potential determinants of the household transmission of SARS-COV-2 were identified on the basis of prespecified characteristics and studies with sufficient data (Table S4). Symptomatic index cases were associated with a higher SAR than asymptomatic index cases (RR = 2.68, 95% CI: 1.39–3.58, I2 = 94%). In different family relationships, the spouse relationship-to-index case was reported to have a significantly higher SAR than other relationships (RR = 1.78, 95% CI: 1.25–2.53, I2 = 91%), whereas the same trend was not shown in the parent-child relationship (RR = 0.84, 95% CI: 0.59–1.19, I2 = 87%). Household contacts with comorbidities were at a higher risk for secondary infections than those without comorbidities (RR = 1.98, 95% CI: 1.52–2.59, I2 = 63%). In terms of sex, the female contacts were observed to be slightly more susceptible than the male contacts (RR = 1.08, 95% CI: 1.01–1.16, I2 = 42%). Another important factor was the household size: a larger household size might be associated with a lower SAR (>4 was <4 members: RR = 0.69, 95% CI: 0.55–0.85, I2 = 94%; >6 vs <6 members: RR = 0.69, 95% CI: 0.50–0.95, I2 = 90%).

Discussion

Analyses of the household transmission of SARS-COV-2 will certainly facilitate a better understanding of the transmission chain and contribute to the epidemic control. Many studies have been conducted on household SAR of SARS-COV-2 (Fung et al., 2021; Koh et al., 2020; Li et al., 2021; Madewell et al., 2020; Shah et al., 2020; Thompson et al., 2021), but only a minority focused on the child group. Irfan et al. (2021) and Zhu et al. (2021) performed meta-analyses on the role of children in household transmission in the early periods of the epidemic, but the results were still unclear because of the limited number of included studies and pediatric index cases. On the basis of previous research, more articles were included in our study. With more timely articles, more comprehensive analyses were conducted. Other than the total pooled household SAR of child contacts and index cases, subgroup analyses were also performed in different SARS-CoV-2 variants and different periods, as well as the transmissibility comparison between child and adult contacts. To the best of our knowledge, almost no previous meta-analyses have been conducted on the pediatric household transmission of different SARS-CoV-2 variants.

Our results show that both the child index cases and secondary cases only comprised a small proportion of the household transmission in case analyses, which suggested that children were unlikely to be the main source of SARS-COV-2 in familial clusters. In the total unclassified results of SAR meta-analyses, lower household transmissibility was demonstrated in both pediatric index cases and contacts than in adults. This was consistent with these previous meta-analyses (Grijalva et al., 2020; Madewell et al., 2021, 2020; Zhu et al., 2021). These findings imply that children are less vulnerable to SARS-COV-2 than adults. Similar to what previous data have shown, the older adults also had a higher SAR than the young adults. Contrary to the analysis by Zhu et al. (2021), a significant difference was found between children younger than and older than 10 years in our analyses, and a recent population-based cohort study also suggested a higher transmissibility of SARS-COV-2 in younger children than older children (Paul et al., 2021). However, this difference still lacked statistical power because of the limited included studies and relatively little advantage, and negative results were also noted in our comprehensive analyses. Therefore, future studies are still required.

Notably, some new findings were found in the subgroup analyses on household SAR of different periods and SARS-COV-2 variants. In the early period of the pandemic (the Wild type mainly dominated during 2019–2020), a relatively low household SAR was observed in children (10–30%), and child contacts usually had lower transmissibility than adults. However, with the emergence of some new variants (Alpha and Delta) in the beginning of 2021, household SAR in children seemed to increase (30–40%). Consistent with our results, many epidemiologic studies have pointed out that children and adolescents had become more susceptible to these new variants (Allen et al., 2022; Chun et al., 2022; Li et al., 2022; Ng et al., 2021; Paul et al., 2021). At the end of 2021, the Omicron variant emerged with the highest transmissibility so far: household SAR in both children and adults seemed to be more than 50%. Plenty of recent research also reported that the rapid increase in infections and hospitalizations was caused by the Omicron variant (Baker et al., 2022; Cloete et al., 2022; Elliott et al., 2022; Marks et al., 2022). Additionally, no significant difference was found in household SAR comparison between children and adults with new variants in our analyses, which also supported the increased vulnerability in children. This was in line with the result of a newly published meta-analysis conducted by Viner et al. (2022). Some research attributed the increased transmissibility to immune escape and reduced effectiveness of vaccination (Meng et al., 2022; Mlcochova et al., 2021; Planas et al., 2021). However, data have proven the protective effect of vaccination even in new variant periods (Fowlkes et al., 2022; Harris et al., 2021; Prunas et al., 2022). Limited by insufficient data, the subgroup analysis on vaccination status was not conducted and the number of articles included in variants analyses was also few. Therefore, original studies that include more virologic data and information on the vaccination status of the participants are still necessary for more convincing results.

Interpretation of the results in the determinant assessment should be more conservative in consideration of the high heterogeneity. A higher SAR was observed in the symptomatic index cases than in asymptomatic. Extensive evidence has proved that mild or asymptomatic patients are less contagious than those with typical clinical symptoms (Cevik et al., 2021; Heald-Sargent et al., 2020; Luo et al., 2020). A larger household size might be associated with a lower SAR. One possible reason may be that large families usually have a low average age and young people tend to be less susceptible. The spouse relationship emerged as a susceptible group in our result. Chaw et al. (2020) suggested that intimate relationships with frequent interaction and prolonged proximity in a closed environment were risk factors. However, negative outcome occurred in the parent-child relationship, which might result from the children's low vulnerability. Household contacts with comorbidities or female contacts were found to be more susceptible, which was also reported in many large population studies (Lyngse et al., 2022; Prunas et al., 2022).

There are several limitations of our systematic review and meta-analysis. First, because the articles included in case analyses were limited and relatively insufficient, larger data sets or more scientific methods are necessary for a more accurate prevalence assessment. In meta-analyses, some included studies were of the retrospective or cross-sectional type, and the information of index cases and contacts was mainly obtained from contact-tracing data sets. Therefore, the determination of the case status might be uncertain, especially the asymptomatic child index cases, which were often mistakenly identified as secondary cases, distorting transmission pathways. The epidemiologic information was self‐reported and subject to recall bias and response bias. In addition, the SAR would be overestimated for not excluding infection resource outside the household and was also underestimated in studies in which only the symptomatic contacts were tested. Because of data insufficiency, many other potential determinants associated with the SAR were not investigated in detail, such as the incubation and infectious periods and public lockdown policy; subgroup analyses of child index cases were also not conducted. Last and most importantly, high unexplained heterogeneity in our analyses constituted an important obstacle when interpreting the results. This might be attributed to the great variation in the design of studies: different definitions of index cases and contacts, inconsistent testing protocols and follow-up time, sociodemographic factors, and so on. Many previous meta-analyses on SAR also ran into the same dilemma (Irfan et al., 2021; Madewell et al., 2020; Shah et al., 2020; Zhu et al., 2021). All of these implied a multitude of related factors and substantial differences among populations. Therefore, the generalizability of our results is limited; compared with the quantitative results, the qualitative conclusions might be more reliable.

Conclusion

Although children were demonstrated to be not dominant in the household transmission, their transmissibility of SARS-CoV-2 appeared to increase as new variants emerged. Given the potentially serious complications of pediatric COVID-19, vaccination research and implementation in children remain a must.