Prolonged Evaluation of COVID-19 Vaccine Effectiveness in Healthcare Workers at a Public Tertiary Hospital: A Retrospective Cohort Study

BACKGROUND: Most studies evaluating COVID-19 vaccine efficacy and effectiveness were conducted over short follow-up periods due to the urgency of vaccine approval and rollout during the pandemic. This study aimed to assess the long-term effectiveness of COVID-19 vaccines among healthcare workers at a tertiary hospital. METHODS: We conducted an open retrospective cohort study at Hospital das Clínicas de Ribeirão Preto, including healthcare workers employed between July 3, 2020, and February 8, 2022. A total of 7,385 workers were followed for 3,185,239 person-days. Three monovalent vaccines were administered during the study period: Coronavac™ (Sinovac), Covishield™ (AstraZeneca), and Comirnaty™ (Pfizer-BioNTech). Participants were categorized as unvaccinated, partially vaccinated, or fully vaccinated. The unadjusted relative risk (RR) of COVID-19 was calculated for vaccinated groups compared with the unvaccinated group. RESULTS: For COVID-19 of any severity, the RR was 0.72 (95%CI: 0.59-0.89) among partially vaccinated and 1.30 (95%CI: 1.19-1.44) among fully vaccinated workers. For severe or critical cases, the RR was 0.75 (95%CI: 0.26-2.16) with partial vaccination and 0.14 (95%CI: 0.05-0.41) with full vaccination. For COVID-19-related deaths, the RR with full vaccination was 0.32 (95%CI: 0.03-3.09). CONCLUSIONS: The vaccines provided limited long-term protection against SARS-CoV-2 infection and mild COVID-19. However, protection against severe disease and death remained substantial despite the circulation of multiple variants.