Risk of Severe Acute Respiratory Syndrome Coronavirus 2 Acquisition Is Associated With Individual Exposure but Not Community-Level Transmission

Abstract

Background: Transmission rates after exposure to a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-positive individual within households and healthcare settings varies significantly between studies. Variability in the extent of exposure and community SARS-CoV-2 incidence may contribute to differences in observed rates.

Methods: We examined risk factors for SARS-CoV-2 infection in a randomized controlled trial of hydroxychloroquine as postexposure prophylaxis. Study procedures included standardized questionnaires at enrollment and daily self-collection of midturbinate swabs for SARS-CoV-2 polymerase chain reaction testing. County-level incidence was modeled using federally sourced data. Relative risks and 95% confidence intervals were calculated using modified Poisson regression.

Results: Eighty-six of 567 (15.2%) household/social contacts and 12 of 122 (9.8%) healthcare worker contacts acquired SARS-CoV-2 infection. Exposure to 2 suspected index cases (vs 1) significantly increased risk for both household/social contacts (relative risk [RR], 1.86) and healthcare workers (RR, 8.18). Increased contact time also increased risk for healthcare workers (3-12 hours: RR, 7.82, >12 hours: RR, 11.81, vs ≤2 hours), but not for household/social contacts. County incidence did not impact risk.

Conclusions: In our study, increased exposure to SARS-CoV-2 within household or healthcare settings led to higher risk of infection, but elevated community incidence did not. This reinforces the importance of interventions to decrease transmission in close contact settings.

Keywords: COVID-19; SARS-CoV-2; healthcare worker transmission; household transmission.

Figure 1.

Study and county daily incidence rates for household/social contacts. A, Study incidence rates per participant, by calendar day, in counties where participants resided with smoothed curve and 95% confidence interval (Poisson generalized additive model). B, Mean modeled county incidence rates per resident, by calendar day, with smoothed loess curve. C, Smoothed curves for study (red) and county (blue) incidence rates with y-axes normalized to the mean. D, Ratio of smoothed study to county incidence rates. While county incidence rates mirrored the overall high incidence seen in the spring of 2020 and lower incidence seen in the summer of 2020 in most areas of the United States, study incidence rates decreased over time.

Figure 2.

Study and county daily incidence rates for healthcare worker contacts. A, Study incidence rates per participant, by calendar day, with smoothed curve and 95% confidence interval (Poisson generalized additive model). B, Mean modeled county incidence rate per resident, by calendar day, with smoothed loess curve. C, Smoothed curves for study (red) and county (blue) incidence rates with y-axes normalized to the mean. D, Ratio of smoothed curves for study and county incidence rates.