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.

Effectiveness of 2 and 3 mRNA COVID-19 Vaccines Doses against Omicron and Delta-Related Outpatient Illness among Adults, October 2021 – February 2022

“…Second, participants who reported a high-risk exposure in the 14 days before illness onset demonstrated lower 3-dose VE during the Omicron predominant period compared with overall VE during this time. These results are consistent with previous studies, including an analysis of data from the US Flu VE Network during the Delta-predominant period [6, 29, 30]. Third, the proportion of participants reporting previous laboratory confirmed SARS-CoV-2 infection was higher when the Omicron variant predominated than when the Delta variant predominated.…”

Antigen Test Positivity After COVID-19 Isolation — Yukon-Kuskokwim Delta Region, Alaska, January–February 2022

“…Therefore, lower prevalence of positive test results over time and after asymptomatic infections might reflect lower infectiousness. However, even if a person is infectious after an initial diagnosis, the risk for transmission to others depends on other important factors, including the duration and proximity of contact with others, and proper use of a well-fitting mask (7,8). Following CDC recommendations, § § YKHC advised continued mask use for 10 days after symptom onset even if a person met the criteria for ending isolation after 5 days.…”

Limitations of Molecular and Antigen Test Performance for SARS-CoV-2 in Symptomatic and Asymptomatic COVID-19 Contacts

“…Our findings suggest that for asymptomatic close contacts with a high pretest probability for infection, testing at a single time point with a standard reference NAT misses some cases, and negative antigen testing does not exclude transmissibility. SARS-CoV-2 exposures that are to household members, indoors, and prolonged pose a higher risk of transmission ( 31 ), which may warrant additional vigilance, including repeat testing for close contacts with high pretest probability for infection. Newer SARS-CoV-2 variants, including delta and omicron, have higher capacity for transmission than earlier circulating genotypes ( 32 , 33 ).…”