Indirect vaccine effectiveness in an outbreak of Alpha B.1.1.7 variant in a California state Prison, May 2021.

Incarcerated populations experienced high rates of SARS-CoV-2 infection and death during early phases of the COVID-19 pandemic. To evaluate vaccine effectiveness in the carceral context, we investigated the first outbreak of COVID-19 in a California state prison following widespread rollout of vaccines to residents in early 2021. We identified a cohort of 733 state prison residents presumed to be exposed between May 14 and June 22, 2021. 46.9 % (n = 344) were vaccinated, primarily with two doses of mRNA-1273 (n = 332, 93.6 %). In total, 92 PCR-positive cases were identified, of which 14 (14.5 %) occurred among mRNA-1273 vaccinated residents. No cases required hospitalization. All nine isolates collected belonged to the Alpha (B.1.1.7) variant. We used Cox proportional hazard regression to estimate vaccine effectiveness for at least one dose of any vaccine at the start of the outbreak. Vaccine effectiveness was 86 % (95 % CI: 75 %-97 %) against PCR-confirmed infection, with similar results for symptomatic infection. Higher rates of building-level vaccine uptake were associated with a lower overall rate of PCR-confirmed infection and symptomatic infection among unvaccinated residents. Among unvaccinated residents who lived in shared cells at the time of presumed exposure, exposure to a vaccinated cellmate was associated with a 38% (95% CI: 0.37, 1.04) lower hazard rate of PCR-confirmed infection over the study period. In this outbreak involving the Alpha SARS-CoV-2 variant, vaccination conferred direct and possibly indirect protection against SARS-CoV-2 infection and symptomatic COVID-19. Our results support the importance of vaccine uptake in mitigating outbreaks and severe disease in the prison setting and the consideration of community vaccination levels in policy and infection response.