Post-Vaccination COVID-19 Infection among Health Care Workers: Description, Determinants, and Event-History Analysis

Background: Healthcare workers (HCWs) are at increased risk of COVID-19 exposure. Although vaccination reduces the risk of infection and its severity, breakthrough infections may still occur due to individual, and occupational factors. This study assessed the incidence, determinants, and time to post-vaccination COVID-19 infection among HCWs in a tertiary care hospital in Bhopal, India. Methods: An Ambi-directional cohort study was conducted from September 2021 to July 2022 with a 6-month follow-up after the second vaccine dose. An event-history (time-to-event) approach using Cox proportional hazards regression was applied to evaluate determinants, adjusting for temporal variations in infection risk. Results: The incidence density of post-vaccination infection was 10.64 per 100 person-years (95% CI: 7.53–13.75). Doctors (27.2) and nurses (15.7) had higher incidence densities than other staff. The median time to infection was 76 days. Older age (>30 years) was associated with lower risk (adjusted HR = 0.35, 95% CI: 0.15–0.79), while longer working hours increased risk (adjusted HR = 1.28, 95% CI: 1.07–1.52). Immediate care providers showed an increased risk, though not statistically significant. All infections were mild, with no hospitalizations. Conclusions: Post-vaccination COVID-19 infections among HCWs were infrequent and mild, with risk varying by occupational exposure. These findings highlight the need for continued risk mitigation in higher-exposure groups while reinforcing the protective effect of vaccination against severe disease.