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

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Key Points

Question  Are viral variants of concern and increased vaccination associated with SARS-CoV-2 household transmission rates?

Findings  In this systemic review and meta-analysis of 135 studies with more than 1.3 million participants in 36 countries, household secondary attack rates increased over time and were higher for Omicron (42.7%), Alpha (36.4%), and Delta (29.7%) variants than previously reported estimates (18.9%). Full vaccination reduced susceptibility and infectiousness, but more so for Alpha than Delta and Omicron.

Meaning  These findings suggest vaccination for SARS-CoV-2 transcends protection of the individual by conferring indirect protection to other household members, but the degree of protection is seemingly lower for emerging variants.


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.

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Article Information

Accepted for Publication: March 10, 2022.

Published: April 28, 2022. doi:10.1001/jamanetworkopen.2022.9317

Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2022 Madewell ZJ et al. JAMA Network Open.

Corresponding Author: Zachary J. Madewell, PhD, Department of Biostatistics, University of Florida, PO Box 117450, Gainesville, FL 32611 (

Author Contributions: Dr Madewell had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Madewell, Longini, Dean.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: Madewell.

Critical revision of the manuscript for important intellectual content: All authors.

Statistical analysis: Madewell, Yang, Longini, Halloran.

Obtained funding: Longini, Dean.

Administrative, technical, or material support: Longini.

Supervision: Dean.

Conflict of Interest Disclosures: Dr Yang reported receiving grants from National Institutes of Health during the conduct of the study. No other disclosures were reported.

Funding/Support: This work was supported by the National Institutes of Health (grant R01-AI139761 to Dr Dean).

Role of the Funder/Sponsor: The funder had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

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