Predictors of Severe Acute Respiratory Syndrome Coronavirus 2 Infection Following High-Risk Exposure

Abstract

Background: Non-pharmaceutical interventions (NPIs) are recommended for COVID-19 prevention. However, the effectiveness of NPIs in preventing SARS-CoV-2 transmission remains poorly quantified.

Methods: We conducted a test-negative design case-control study enrolling cases (testing positive for SARS-CoV-2) and controls (testing negative) with molecular SARS-CoV-2 diagnostic test results reported to California Department of Public Health between 24 February-12 November, 2021. We used conditional logistic regression to estimate adjusted odds ratios (aORs) of case status among participants who reported contact with an individual known or suspected to have been infected with SARS-CoV-2 ("high-risk exposure") ≤14 days before testing.

Results: 751 of 1448 cases (52%) and 255 of 1443 controls (18%) reported high-risk exposures ≤14 days before testing. Adjusted odds of case status were 3.02-fold (95% confidence interval: 1.75-5.22) higher when high-risk exposures occurred with household members (vs. other contacts), 2.10-fold (1.05-4.21) higher when exposures occurred indoors (vs. outdoors only), and 2.15-fold (1.27-3.67) higher when exposures lasted ≥3 hours (vs. shorter durations) among unvaccinated and partially-vaccinated individuals; excess risk associated with such exposures was mitigated among fully-vaccinated individuals. Cases were less likely than controls to report mask usage during high-risk exposures (aOR = 0.50 [0.29-0.85]). The adjusted odds of case status was lower for fully-vaccinated (aOR = 0.25 [0.15-0.43]) participants compared to unvaccinated participants. Benefits of mask usage were greatest among unvaccinated and partially-vaccinated participants, and in interactions involving non-household contacts or interactions occurring without physical contact.

Conclusions: NPIs reduced the likelihood of SARS-CoV-2 infection following high-risk exposure. Vaccine effectiveness was substantial for partially and fully vaccinated persons.

Keywords: SARS-CoV-2; face masks; non-pharmaceutical interventions; vaccination.

Figure 1.

Predictors of infection following high-risk exposure. aORs computed using conditional logistic regression models interacting vaccination status with each contact attribute and adjusting for community exposures (listed in the main text), vaccination status (defined as fully vaccinated or unvaccinated/incompletely vaccinated) of the participant, mask-wearing by the participant and their contact, level of anxiety about coronavirus disease 2019 prior to testing, and participants’ age, sex, and region of residence. Regression strata were defined for county reopening tiers and, for the period after 15 June, the month of severe acute respiratory syndrome coronavirus 2 test. Further regression parameter estimates are presented in Supplementary Table 4. Counts for cases and controls differ from those listed in Table 1 because some participants indicated that they did not know these details about their known or suspected contact and because of missing data on vaccination status among cases (N = 8) and controls (N = 18). Abbreviations: aOR, adjusted odds ratio; CI, confidence interval.

Figure 2.

Protective effects of mask-wearing and vaccination in the context of high-risk exposure. aORs computed using conditional logistic regression models adjusting for vaccination status, community exposures (listed in the main text), characteristics of high-risk contact, level of anxiety about coronavirus disease 2019 prior to testing, and participants’ age, sex, and region of residence. Regression strata were defined for county reopening tiers and week of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) test. An individual was considered fully vaccinated if their SARS-CoV-2 test date was >14 days after their second dose of an mRNA vaccine product (Pfizer/BioNTech BNT-162b2 or Moderna mRNA-1273) or >14 days after their first dose of a single-dose product (Jansen Pharmaceutical Companies JNJ-78436735). In sensitivity analyses limiting to those who received an mRNA vaccine product (excluding N = 25 recipients of JNJ-78436735), the aORs (95% CIs) for incompletely vaccinated and fully vaccinated individuals were 0.30 (.14–.63) and 0.26 (.14–.46), respectively. Abbreviations: aOR, adjusted odds ratio; CI, confidence interval.

Figure 3.

Protective effects of mask-wearing in differing high-risk exposure contexts. aORs computed using conditional logistic regression models adjusting for vaccination status of respondent, community exposures (listed in main text), characteristics of the high-risk contact, level of coronavirus disease 2019 anxiety prior to testing, and participants’ age, sex, and region of residence. An interaction term was included between mask usage and the contact attribute in 5 separate models. Regression strata were defined for county reopening tiers and week of severe acute respiratory syndrome coronavirus 2 test. The aOR represents the aOR for case status comparing mask usage within each category (with respect to relationship, physical/nonphysical nature of contact, indoor/outdoor exposure, duration, and participant vaccination status). Abbreviations: aOR, adjusted odds ratio; CI, confidence interval.