Effectiveness of COVID-19 vaccine in health care workers, Milan, Italy

ABSTRACT

Introduction Randomized controlled trials showed efficacy of vaccines against coronavirus disease 19 (COVID-19). There is the need to quantify vaccine effectiveness in real-word contexts, including people at high risk of infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), such as health care workers (HCWs). Objectives To evaluate vaccine effectiveness among hospital HCWs. Methods We performed a cohort study among HCWs of a large University hospital in Milan, Lombardy, Italy by merging routinely collected data on demographics, COVID-19 vaccination, and polymerase chain-reaction (PCR) tests performed on nasopharyngeal swabs. Follow-up started on December 27, 2020 (start of vaccination campaign). We included HCWs never PCR-positive before the start date and with at least a PCR test afterwards. Vaccination was treated as a time-dependent variable by calculating person-years (PY) at risk before and after vaccine doses. Subjects contributed PY until first positive PCR test (cases) or last test for never positive HCWs (to avoid immortal time bias). We calculated infection rates (cases per 1000 PY), rate ratios (RR, with a Poisson regression model adjusted for gender, age, occupation and 30-day periods), vaccine effectiveness (VE = (1-RR)x100) and 95% confidence intervals (CI) taking never vaccinated HCWs as reference. Results As of May 10, there were 3,152 vaccinated (97% with BNT162b2, 140 with one dose, 2,679 with two doses) and 333 non-vaccinated. We counted 29 infected cases (rate 385) among non-vaccinated, 6 (rate 65) from day 14 after the first dose (VE 79%, CI 49-92%), and 24 (rate 65) from day 7 after the second dose (VE 89%, CI 80-94%). Most cases after vaccination were asymptomatic or mildly symptomatic. Conclusion In these preliminary analysis we found high effectiveness of COVID-19 vaccine in HCWs in our hospital. Further work is needed to assess long-term effectiveness and to better plan future preventive strategies among this high-risk occupational group.