Recent COVID-19 Vaccination and Risk of SARS-CoV-2 Transmission.

Importance: COVID-19 vaccine effectiveness (VE) is typically studied in the context of reducing the risk of severe illness and death. Few studies have estimated VE in preventing transmission and infection with current levels of SARS-CoV-2 population immunity. Objective: To estimate COVID-19 VE against transmission and infection within households. Design, Setting, and Participants: This cohort study was a prospective, case-ascertained household transmission study (performed in New York, Tennessee, and Washington) in which the first household member with confirmed SARS-CoV-2 infection (primary case participant) was identified through outpatient settings and enrolled with their household contacts from January 1, 2024, to January 31, 2025. Participants provided demographic information, and COVID-19 vaccination history was verified by study staff. After enrollment, participants were instructed to collect daily nasal swabs for 10 days regardless of symptoms. Nasal swabs were tested for SARS-CoV-2 via reverse transcription-polymerase chain reaction. Exposure: COVID-19 vaccination history in primary case participants and household contacts categorized as time from most recent vaccination to COVID-19 onset in the primary case participant (≤6 months, 7-12 months, >12 months, and unvaccinated [reference group]). Main Outcomes and Measures: Household contacts were considered infected if at least 1 swab tested positive for SARS-CoV-2. Secondary infection risk was calculated as the number of infected contacts divided by the total number of contacts. Adjusted relative risk (ARR) of infection was estimated using a multivariable Poisson regression model, with generalized estimating equations accounting for household-level clustering. Vaccine effectiveness was calculated as 1 minus the ARR of the primary case participant and household contacts' vaccination status to estimate VE against transmission and against infection, respectively. Results: This analysis included 362 primary case participants (median [IQR] age, 35 [10-53] years; 199 female [55.0%]) and 763 household contacts (median [IQR] age, 29 [12-44] years; 399 female [52.3%]). SARS-CoV-2 infection was detected in 476 household contacts during follow-up for a secondary infection risk of 62.4% (95% CI, 58.7%-65.5%). Household contacts of primary case participants vaccinated 6 months or less before onset had a lower infection risk compared with contacts of unvaccinated primary case participants (ARR, 0.57 [95% CI, 0.35-0.93]). There was no statistically significant difference in infection risk based on vaccination status of household contacts. Conclusions and Relevance: In this cohort study, recent COVID-19 vaccination was associated with a reduced risk of SARS-CoV-2 transmission. These findings suggest that COVID-19 vaccination may have an indirect benefit of decreasing transmission and thus reducing overall exposure to SARS-CoV-2.