Interim Vaccine Effectiveness Against Influenza and Hospitalization, Republic of Korea, 2024-2025 (HIMM Network)

Background: Influenza cases have surged earlier than usual during the 2024-2025 season, with A/H1N1 (pdm09) being the dominant strain. We aimed to investigate early estimates of influenza vaccine effectiveness (VE) for the 2024-2025 season to enhance our influenza response strategies. Methods: From November 1 to December 31, 2024, we enrolled 990 individuals with influenza-like illness from the hospital-based influenza surveillance network (Hospital-Based Influenza Morbidity and Mortality, HIMM), which consists of eight hospitals. Results: The overall adjusted VE was estimated to be -0.5% (95% confidence interval [CI], -34.0 to 24.6), with 0.4% (95% CI, -33.2 to 25.5) for influenza A. Analyses by influenza subtype were exploratory, given the limited number of subtyped cases. Although ineffective in preventing laboratory-confirmed influenza, influenza vaccination reduced influenza-related hospitalizations by 31.9% (95% CI, 3.5 to 51.9). Conclusions: It is necessary to enhance influenza vaccine effectiveness by selecting better-matched vaccine strains and introducing immune-enhanced vaccines.