Incidence and risk factors of COVID-19 in a tertiary hospital and the effectiveness of booster vaccination among health care workers: A retrospective cohort study, January 2020 to June 2022

Background: Health care workers (HCWs) face a higher risk of infection and may transmit pathogens to patients during a pandemic. This study aims to evaluate infection-control measures by analyzing the incidence and risk factors of COVID-19 and estimating vaccine effectiveness (VE) at a tertiary hospital in Seoul, Republic of Korea. Methods: This study included 2,516 HCWs from January 1, 2020, to June 30, 2022. Data were analyzed to determine the incidence density and cumulative incidence; the results were compared by the age- and gender-specific standardized incidence ratios (SIR). VE was estimated with multivariate Cox proportionalhazard models as 1-adjusted hazard ratio x 100%. Results: SIR indicated a lower COVID-19 risk in the hospital population than in the general Korean population (SIR, 0.81; 95% confidence interval [CI]: 0.76-0.87). Multivariate Cox analysis indicated that, compared to doctors, nonmedical service supporters and other HCWs (excluding doctors and nurses) were high-risk groups (adjusted hazard ratio [95% CI], 1.72 [1.04-2.83] and 1.76 [1.20-2.58], respectively). Compared to the outpatient unit, the emergency department was a high-risk department (1.70 [1.16-2.50]). The VE of the booster dose was 55.47%, compared to no or incomplete vaccination (95% CI: 22.63-74.37). Conclusions: Besides encouraging HCWs vaccination, effective infection-control measures should target high-risk groups and departments. (c) 2024 The Author(s). Published by Elsevier Inc. on behalf of Association for Professionals in Infection Control and Epidemiology, Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).