The Household Secondary Attack Rate of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2): A Rapid Review

Abstract

Background: Although much of the public health effort to combat coronavirus disease 2019 (COVID-19) has focused on disease control strategies in public settings, transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within households remains an important problem. The nature and determinants of household transmission are poorly understood.

Methods: To address this gap, we gathered and analyzed data from 22 published and prepublished studies from 10 countries (20 291 household contacts) that were available through 2 September 2020. Our goal was to combine estimates of the SARS-CoV-2 household secondary attack rate (SAR) and to explore variation in estimates of the household SAR.

Results: The overall pooled random-effects estimate of the household SAR was 17.1% (95% confidence interval [CI], 13.7-21.2%). In study-level, random-effects meta-regressions stratified by testing frequency (1 test, 2 tests, >2 tests), SAR estimates were 9.2% (95% CI, 6.7-12.3%), 17.5% (95% CI, 13.9-21.8%), and 21.3% (95% CI, 13.8-31.3%), respectively. Household SARs tended to be higher among older adult contacts and among contacts of symptomatic cases.

Conclusions: These findings suggest that SARs reported using a single follow-up test may be underestimated, and that testing household contacts of COVID-19 cases on multiple occasions may increase the yield for identifying secondary cases.

Keywords: SARS-CoV-2; household transmission; secondary attack; testing frequency.

Figure 1.

Study selection. Excluded records may have had more than 1 reason for exclusion, but only 1 reason was listed for each record. Records from bioRxiv and medRxiv that fell outside the 50 best matches were largely irrelevant (eg, not related to SARS-CoV-2 or examined the economic impact of the pandemic). Abbreviations: COVID, coronavirus disease; RT-PCR, reverse transcription polymerase chain reaction; SAR, secondary attack rate; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 2.

Estimates of the household secondary attack rate, stratified by country and study design. Exact binomial confidence intervals were computed for estimates reported without uncertainty.