Abstract
BACKGROUND: A SARS-CoV-2 outbreak was detected in a nursing home in February 2021 after residents and staff had received two doses of BNT162b2 vaccine in January 2021. METHODS: Nursing home staff, long-term residents and day-care receivers were included in a retrospective cohort study. We calculated attack rates (AR), secondary AR (SAR) and their 95% binomial confidence interval (CI), and we compared them using Fisher's exact test or chi-squared test, depending on the sample size. We used Poisson regression with robust error estimates to calculate vaccine effectiveness against SARS-COV-2 infections. We selected variables based on directed acyclic graphs. As a proxy for viral load at diagnosis, we compared the mean Ct values at diagnosis using t tests or Mann-Whitney U tests. RESULTS: The adjusted vaccine effectiveness against infection was 56% (95% CI: 15-77%, p = 0.04). Ct values at diagnosis were higher when intervals after receiving the second vaccination were longer (>21 vs. ≤21 days: 4.48 cycles, p = 0.08). The SAR was 67% lower in households of vaccinated (2/9 [22.2%]) than of unvaccinated infected staff (12/18 [66.7%]; p = 0.046). Vaccination rates were lowest among staff with close physical contact to care-receivers (46%). The highest AR in vaccinated staff had those working on wards (14%). CONCLUSIONS: Vaccination reduced the risk for SARS-CoV-2 infection, viral load and transmission; however, non-pharmaceutical interventions remain essential to reduce transmission of SARS-CoV-2 infections, even for vaccinated individuals. Vaccination coverage of staff ought to increase reduction of infections among themselves, their household members and residents.