Household Secondary Attack Rates of SARS-CoV-2 by Variant and Vaccination Status: An Updated Systematic Review and Meta-analysis

Abstract

Importance: An overall household secondary attack rate (SAR) of 18.9% (95% CI, 16.2%-22.0%) through June 17, 2021 was previously reported for SARS-CoV-2. Emerging variants of concern and increased vaccination have affected transmission rates.

Objective: To evaluate how reported household SARs changed over time and whether SARs varied by viral variant and index case and contact vaccination status.

Data sources: PubMed and medRxiv from June 18, 2021, through March 8, 2022, and reference lists of eligible articles. Preprints were included.

Study selection: Articles with original data reporting the number of infected and total number of household contacts. Search terms included SARS-CoV-2, COVID-19, variant, vaccination, secondary attack rate, secondary infection rate, household, index case, family contacts, close contacts, and family transmission.

Data extraction and synthesis: The Preferred Reporting Items for Systematic Reviews and Meta-Analyses reporting guideline was followed. Meta-analyses used generalized linear mixed models to obtain SAR estimates and 95% CIs.

Main outcomes and measures: SAR stratified by covariates according to variant, index case and contact vaccination status, and index case identification period. SARs were used to estimate vaccine effectiveness on the basis of the transmission probability for susceptibility to infection (VES,p), infectiousness given infection (VEI,p), and total vaccine effectiveness (VET,p).

Results: Household SARs were higher for 33 studies with midpoints in 2021 to 2022 (37.3%; 95% CI, 32.7% to 42.1%) compared with 63 studies with midpoints through April 2020 (15.5%; 95% CI, 13.2% to 18.2%). Household SARs were 42.7% (95% CI, 35.4% to 50.4%) for Omicron (7 studies), 36.4% (95% CI, 33.4% to 39.5%) for Alpha (11 studies), 29.7% (95% CI, 23.0% to 37.3%) for Delta (16 studies), and 22.5% (95% CI, 18.6% to 26.8%) for Beta (3 studies). For full vaccination, VES,p was 78.6% (95% CI, 76.0% to 80.9%) for Alpha, 56.4% (95% CI, 54.6% to 58.1%) for Delta, and 18.1% (95% CI, -18.3% to 43.3%) for Omicron; VEI,p was 75.3% (95% CI, 69.9% to 79.8%) for Alpha, 21.9% (95% CI, 11.0% to 31.5%) for Delta, and 18.2% (95% CI, 0.6% to 32.6%) for Omicron; and VET,p was 94.7% (95% CI, 93.3% to 95.8%) for Alpha, 64.4% (95% CI, 58.0% to 69.8%) for Delta, and 35.8% (95% CI, 13.0% to 52.6%) for Omicron.

Conclusions and relevance: These results suggest that emerging SARS-CoV-2 variants of concern have increased transmissibility. Full vaccination was associated with reductions in susceptibility and infectiousness, but more so for Alpha than Delta and Omicron. The changes in estimated vaccine effectiveness underscore the challenges of developing effective vaccines concomitant with viral evolution.

Figure 1.. Household Secondary Attack Rates Over Time, by Study Midpoint, in 135 Studies of Unvaccinated Index Cases and Unvaccinated Contacts

Data were restricted to laboratory-confirmed results only. The blue line is a loess smoothing line, and shaded bands are 95% CIs. Bicolored points represent studies with 2 predominant variants.

Figure 2.. Household Secondary Attack Rates (SARs) for Omicron (B.1.1.529), Alpha (B.1.1.7), Delta (B.1.617.2), and Beta (B.1.351) Variants

SARs included all index cases and contacts regardless of vaccination status. Point sizes (squares) are an inverse function of the precision of the estimates, and bars correspond to 95% CIs. Diamonds represent summary SAR estimates with corresponding 95% CIs. Heterogeneity indexes were as follows: Omicron (I² = 98.2%), Alpha (I² = 59.6%), Delta (I² = 99.1%), and Beta (I² = 2.6%).

Figure 3.. Household Secondary Attack Rates (SARs) by Index Case Vaccination Status

All contacts are included regardless of vaccination status. For Harris et al, most of the vaccinated index cases (93%) had received only the first dose of vaccine and SARs were not disaggregated by dose. Point sizes (squares) are an inverse function of the precision of the estimates, and bars correspond to 95% CIs. Diamonds represent summary SAR estimates with corresponding 95% CIs. Heterogeneity indexes are as follows: Alpha (unvaccinated: I² = 94.6%; fully vaccinated: I² = 52.7%), Delta (unvaccinated: I² = 99.1%; partially vaccinated: I² = 91.7%; fully vaccinated: I² = 98.6%), and Omicron (unvaccinated: I² = 93.5%; partially vaccinated: I² = 70.5%; fully vaccinated: I² = 0.4%; booster vaccinated: I² = 78.5%).

Figure 4.. Household Secondary Attack Rates (SARs) by Contact Vaccination Status

All index cases are included regardless of vaccination status. Point sizes (squares) are an inverse function of the precision of the estimates, and bars correspond to 95% CIs. Diamonds represent summary SAR estimates with corresponding 95% CIs. Heterogeneity indexes are as follows: Alpha (unvaccinated: I² = 92.5%; fully vaccinated: I² = 69.5%), Delta (unvaccinated: I² = 95.4%; partially vaccinated: I² = 30.4%; fully vaccinated: I² = 96.8%; booster vaccinated: I² = 76.5%), and Omicron (unvaccinated: I² = 97.1%; partially vaccinated: I² = 0.3%; fully vaccinated: I² = 60.1%; booster vaccinated: I² = 92.5%).