Boosters protect against SARS-CoV-2 infections in young adults during an Omicron-predominant period

Background: While booster vaccinations clearly reduce the risk of severe COVID-19 and death, the impact of boosters on SARS-CoV-2 infection has not been fully characterized: doing so requires understanding their impact on asymptomatic and mildly symptomatic infections that often go unreported but nevertheless play an important role in spreading SARS-CoV-2. We sought to estimate the impact of COVID-19 booster doses on SARS-CoV-2 infection in a vaccinated and actively surveilled population of young adults during an Omicron-predominant period. Methods and Findings: We implemented a cohort study of young adults in a college environment (Cornell University's Ithaca campus) from December 5, 2021 through December 31, 2021 when Omicron was deemed the predominant SARS-CoV-2 variant on campus. Participants included 15,102 university students fully vaccinated with an FDA-authorized or approved vaccination (BNT162b2, mRNA-1273, or Ad26.COV2.S) who were enrolled in mandatory at-least-weekly surveillance PCR testing, and having no positive SARS-CoV-2 PCR test within 90 days before the start of the study period. Multivariable logistic regression considering those with full vaccination (a 2-dose series of BNT162b2 or mRNA-1273, or 1 dose of Ad26.COV2.S) with a booster dose versus those without a booster dose. 1,870 SARS-CoV-2 infections were identified in the study population. Controlling for gender, student group membership, full vaccination date and initial vaccine type, our analysis estimates that receiving a booster dose reduces the odds of having a PCR-detected SARS-CoV-2 infection relative to full vaccination by 52% (95% confidence interval [37%, 64%]). This result is robust to the choice of the delay over which a booster dose becomes effective (varied from 1 day to 14 days). Conclusions: Boosters are effective, relative to full vaccination, in reducing the odds of SARS-CoV-2 infections in a college student population during a period when Omicron was predominant. Therefore, booster vaccinations for this age group can play an important role in reducing community and institutional transmission.