ABSTRACT
Introduction: Registration studies have shown high efficacy of BNT162b2 mRNA COVID-19 vaccine. We evaluated vaccine effectiveness (VE) of BNT162b2 mRNA COVID-19 vaccine in a cohort of healthcare workers (HCWs) of a large hospital in Milan, Lombardy, Italy. Material and Methods: Follow-up started on 27 December 2020 (beginning of the vaccination campaign). HCWs without history of SARS-CoV-2 infection before the start date and with at least a nasopharyngeal test afterwards were included. Vaccination was treated as a time-dependent variable. For selected periods after vaccination we calculated incidence rate ratios (IRR) and 95% confidence intervals (CI) of infection with a Poisson regression model adjusted for gender, age, occupation, and 30-day periods, and then VE as (1 – IRR)x100 using unvaccinated person-time as reference. Databases were closed on 27 September 2021. The study was approved by the hospital’s ethics committee (Milano Area 2, Prot. No. 828_2021bis). Results and Conclusions: We included 3,809 HCWs, 131 still unvaccinated and 3,678 vaccinated (3,576 with two doses). We identified 134 infections (62% symptomatic). Adjusted VE was 77% (CI: 43-91) from day 14 after the first vaccine dose and 87% (CI: 79-92) at least 7 days after the second dose. After full vaccination schedule VE was 89% (CI: 82-94) for symptomatic and 77% (CI: 45-90) for asymptomatic infections. In conclusion, we found high effectiveness of BNT162b2 vaccine in reducing incidence of both symptomatic and asymptomatic infections. The follow-up is continuing to assess long-term effectiveness, also considering emerging SARS-CoV-2 variants.