Influenza Vaccine Effectiveness in Korean Children: A Registry-Linked, Survey-Based Population Study (2023–2024)

Background: Most estimates of vaccine effectiveness (VE) against seasonal influenza (SI) in children are based on test-negative design studies in hospital-based settings, limiting their generalizability to the broader community.In South Korea, nationwide populationbased survey data enable community-level assessments of VE.This study aimed to estimate the incidence and VE of the SI vaccine among Korean children aged 5-13 years during the 2023-2024 season.Methods: This study used data from the fifth Korea Seroprevalence Study of Monitoring of SARS-CoV-2 Antibody Retention and Transmission (K-SEROSMART), linked to the national influenza vaccination registry.We included 406 children aged 5-13 years who were eligible for Korea's National Immunization Program and participated in the June 2024 K-SEROSMART survey.The primary outcomes were 1) self-reported test-or provider-diagnosed influenza and 2) influenza-like illness (ILI) (fever ≥ 38°C plus respiratory symptoms).Weighted cumulative incidence and adjusted VE were estimated using survey-weighted regression models.Results: Among 406 children (mean age, 9.5 years; 49.5% female), vaccine coverage was 70.0% (284/406).The weighted cumulative incidence was 14.4% (95% confidence interval [CI], 10.4-18.4) for self-reported test-or provider-diagnosed influenza and 23.6% (95% CI, 19.0-28.2) for ILI.The VE was estimated at 41% (95% CI, -10 to 68) against test-or providerdiagnosed influenza and 4% (95% CI, -91 to 52) against ILI.Conclusion: The 2023-2024 influenza vaccine was associated with a numerically lower risk of influenza diagnosis among South Korean children; however, the estimates were imprecise and not statistically significant, with minimal protection against ILI.These findings underscore the need for continued monitoring of influenza VE in community-based pediatric populations.