Assessment of influenza vaccine effectiveness among the elderly in Taiwan using population-based registry data for the 2023-2024 season.

Introduction: Influenza infection can cause serious complications in the elderly, including hospitalizations and death. In Taiwan, government-funded influenza vaccination is offered to the elderly ≥65 years old. We aim to evaluate vaccine effectiveness (VE) among this group during 2023-2024 influenza season using national databases.

Methods: A retrospective cohort study was conducted to evaluate VE among adults aged ≥65 years. Individuals were grouped by vaccination status: those vaccinated between October 2 and November 15, 2023 comprised the vaccinated group, while those without vaccination records during this period formed the unvaccinated group. The observation period began 14 days after vaccination (index date) and continued through February 15, 2024; for unvaccinated individuals, the index date was October 2, 2023. Outcome and covariate data were obtained from linked national databases. VE was estimated using adjusted logistic regression models, with sensitivity analyses using propensity score matching.

Results: Among 4,174,655 individuals, 1,833,476 (44 %) were vaccinated. Influenza vaccination showed moderate to high effectiveness against laboratory-confirmed influenza outcomes, with adjusted VE ranging from 29.6 % (outpatient visits) to 75.2 % (severe disease). Moderate effectiveness was observed against influenza-like illness outpatient visits (VE: 21.2 %), inpatient visits (37.8 %) and all-cause mortality within 30 days (48.2 %). VE estimates increased after covariate adjustment, especially for severe outcomes. Findings were consistent in analyses using propensity score matching.

Conclusions: Influenza vaccination was moderately to highly effective against both laboratory-confirmed influenza and influenza-like illness in elderly adults. Protection was observed across outpatient visits, hospitalizations, and mortality.