Objectives: The first National Influenza Annual Report was jointly released by the Korea Disease Control and Prevention Agency (KDCA) and the National Health Insurance Service (NHIS) to provide a comprehensive analysis of influenza activity and the burden of disease during the 2024-2025 influenza season.

Methods: Influenza activity was analyzed using patient- and pathogen-based surveillance data, medical costs data, vaccination coverage, and vaccine effectiveness (VE) estimates. Data were obtained from the KDCA sentinel surveillance system, the National Immunization Program, and the NHIS national health information databases. VE was estimated using both a target trial emulation (TTE)-based retrospective cohort design to assess age-specific outcomes and an interrupted time series (ITS) method with dynamic cohort to evaluate time-varying protection.

Results: During the 2024-2025 season, influenza-like illness peaked at 99.8 cases per 1,000 outpatients in the first week of 2025, the highest level recorded since 2016. The overall influenza virus detection rate was 15.2%, with a winter wave dominated by influenza A and a spring wave of influenza B. NHIS claims identified approximately 3.86 million influenza cases, indicating a decline from the previous season. Despite this decrease in case numbers, total medical costs continued to increase in the post-coronavirus disease 2019 period, reaching 629.5 billion KRW, with hospitalization accounting for 77.3% of the total costs. Vaccination coverage was 70.0% among children aged 6 months to 13 years and 81.6% among adults aged 65 years and older. The TTE analysis showed that VE against severe outcomes ranged from 63.7% to 74.6%, whereas VE against mortality ranged from 52.2% to 81.1%, depending on the age group. The ITS method demonstrated that protection against hospitalization (VE 41.8%) persisted for up to 2 months after vaccination, whereas protection against mortality (VE 38.1%) persisted for up to 6 months. Overall, influenza vaccination was estimated to have prevented 143,868 influenza cases, including outpatient and inpatient cases, as well as 3,506 deaths.

Conclusions: The findings presented in this annual report provide scientific evidence for optimizing national influenza prevention and response strategies and advancing immunization policy.