Effectiveness of the 2023-2024 monovalent XBB.1.5 vaccine against SARS-CoV-2 infection in people aged over 65 years in South Korea: a population-based cohort study

Objectives The continued emergence of SARS-CoV-2 variants, including XBB.1.5 and its sublineages, poses challenges for COVID-19 control, particularly in high-risk populations. This study evaluated the community-based effectiveness of the 2023-2024 monovalent XBB.1.5 vaccine in preventing SARS-CoV-2 infection among individuals aged 65 years and older in South Korea. Methods This retrospective cohort study analyzed data from the K-SEROSMART, a nationwide seroepidemiological survey. The primary outcome was self-reported COVID-19 infection. To capture undiagnosed cases, this outcome was expanded to include anti-N antibody seroconversion. The follow-up period spanned from October 2023 to July 2024. Vaccine effectiveness was estimated using a Cox model with time-varying exposure for self-reported infection and conditional logistic regression for the expanded outcome based on nested case-control design with 1:4 matching. Results Among 3079 participants, 54.8% received the XBB.1.5 vaccine. The incidence of self-reported COVID-19 infection was lower in the vaccinated group (1.4%) than in the unvaccinated group (2.9%) ( P = 0.01). Estimated vaccine effectiveness was 54% (95% CI: 19-74%) against self-reported infection and 33% (95% CI: 7-51%) using the expanded definition. Conclusion These findings support continued COVID-19 vaccination efforts, particularly in older adults. Regular vaccination remains essential to maintaining immunity and mitigating transmission, guiding future strategies against emerging SARS-CoV-2 variants.