Joint Publication of the First 2024–2025 National Influenza Annual Report by the Korea Disease Control and Prevention Agency and the National Health Insurance Service

Abstract

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.

Key messages

① What is known previously?

Korea Disease Control and Prevention Agency (KDCA) has been monitoring Influenza activity with sentinel surveillance and implemented National Immunization Program to minimize its public health impact. The National Health Insurance Service (NHIS) manages national health insurance benefits for disease treatment and health promotion.

② What new information is presented?

KDCA and the NHIS published the first National Influenza Annual Report of 2024–2025 season by leveraging big data. The report provides a comprehensive overview of influenza activities including patients and pathogen, medical costs, vaccination coverage and vaccine effectiveness.

③ What are implications?

The collaboration between the KDCA and NHIS has expanded the monitoring scope, providing critical scientific evidence to optimize national response strategies and improve immunization policies.

Introduction

Influenza is a seasonal respiratory infection that peaks annually during winter, imposing a substantial burden on both individual health and societal infrastructure. Especially among high-risk groups such as older adults, it markedly increases the risk of severe complications and mortality, leading to considerable socioeconomic burdens and strain on healthcare resources [1]. Furthermore, as the potential for a pandemic driven by viral mutations remains a persistent threat, systematic and continuous surveillance is an essential component of the national public health response [2]. Accordingly, the Korea Disease Control and Prevention Agency (KDCA) has designated influenza as a Class 4 notifiable infectious disease and operates patient and pathogen sentinel surveillance systems to closely monitor influenza activity and enable a timely response [3-5]. In addition, the National Immunization Program (NIP) has been implemented to minimize health impacts due to influenza [6].

The KDCA and the National Health Insurance Service (NHIS) have maintained a close data collaboration system, including the establishment of the “COVID-19 Big Data (K-COV-N),” based on the Memorandum of Understanding signed in April 2021 [7]. To support the efficient management of influenza, which imposes a high disease burden on society, the two organizations expanded their data collaboration to include influenza and, for the first time, published the “2024–2025 Seasonal National Influenza Annual Report,” presenting the key findings¹⁾.

Results

The annual report utilized patient and pathogen sentinel surveillance data, vaccination registry data from KDCA, and healthcare claims data from NHIS. The report included the results of sentinel surveillance, overall influenza incidence and healthcare costs based on health insurance claims data, influenza vaccination coverage under the NIP, and vaccine effectiveness (VE). Analyses for each indicator were conducted by the responsible departments of KDCA and NHIS, and the influenza VE analysis, which employed a target trial emulation–based cohort study design, was performed by external experts as part of a government-commissioned research project.

1. Patient and Pathogen Sentinel Surveillance Results

1) Patient sentinel surveillance results

The influenza-like illness rate for the 2024–2025 season, identified through sentinel surveillance sites at primary care clinics, peaked at 99.8 per 1,000 outpatients in week 1 of 2025. This peak was the highest recorded since the establishment of the current surveillance system in 2016 (Supplementary Figure 1A; available online). By age group, school-aged children and adolescents aged 7–18 years drove the epidemic, and a minor second wave was observed in March, 2025 (Supplementary Figure 1B; available online). According to the hospital-level sentinel surveillance system, the total number of hospitalized influenza cases was 8,640, which was 29.3% lower than that in the previous season; however, the number of cases in the peak week reached 1,632, which was 48.2% higher than that in the previous season. Of these, 4,528 were individuals aged ≥65 years, accounting for 52.4% of total hospitalizations (Supplementary Figure 1C, D; available online).

2) Pathogen sentinel surveillance results

The influenza virus detection rate identified through the Korea Respiratory Virus Integrated Surveillance System was 15.2%, and A(H1N1)pdm09 served as the driver of the epidemic during its initial phase (Supplementary Figure 2A; available online). The age-specific detection rate was the highest among school-aged individuals aged 13–18 years (27.3%), and during the first epidemic period, the detection rate among older adults increased by approximately 20% compared with the previous year (Supplementary Figure 2B; available online). In the spring season, influenza B circulated, and school-aged individuals were again found to have driven the epidemic. Circulating strains were genetically similar to the vaccine strain for that corresponding season, and no resistance to antiviral agents (e.g., Tamiflu) was identified (Supplementary Table 1; available online).

2. Disease Burden

1) Epidemiologic analysis

Approximately 3.86 million influenza cases were identified from health insurance claims, with a sex- and age-standardized incidence rate of approximately 8,200 cases per 100,000 population. The number of cases decreased compared with that in the previous season (Figure 1, Supplementary Figure 3A; available online). The rates of emergency department visits and hospitalizations were the highest among those aged 65 years and older, and these rates tended to increase with age (Supplementary Figure 3B, C; available online).

Figure 1. Number of estimated influenza cases (A) and standardized incidence rates (B) by season.

2) Medical costs

Total medical care benefit expenses for influenza have shown a continuous increasing trend since the coronavirus disease 2019 (COVID-19) pandemic. For the 2024–2025 influenza season, total expenses were KRW 629.5 billion, with inpatient and outpatient care accounting for 77.3% (KRW 486.8 billion) and 22.7% (KRW 142.7 billion), respectively (Supplementary Figure 4A, B; available online).

3. Influenza National Immunization Status and Vaccine Effectiveness Evaluation

1) National immunization status

The national vaccination coverage rate was 70.0% (approximately 3.42 million children) among children aged 6 months to 13 years and 81.6% (approximately 8.39 million adults) among adults aged 65 years and older. Approximately 160,000 pregnant women were vaccinated (Figure 2).

Figure 2. Vaccination coverage under the National Immunization Program for influenza during the 2024–2025 season.

2) Evaluation of vaccine effectiveness

VE was evaluated using health insurance claims data linked to the KDCA vaccination registry to estimate effectiveness against infection, hospitalization, severe disease, and death.

First, the evaluation using a target trial emulation–based retrospective cohort study design matched the case and control groups 1:1 on demographic characteristics, comorbidities, healthcare utilization patterns, and history of confirmed COVID-19 and estimated the preventive effect using the odds ratio²⁾. As a result, VE was estimated at 10.2–41.4%, 4.0–39.2%, 63.7–74.6%, and 52.2–81.1% against infection, hospitalization, severe disease, and death, respectively, depending on the age group (Supplementary Figure 5; available online). However, among younger age groups, the frequency of severe disease and deaths was substantially low (10 or fewer cases), resulting in wide confidence intervals for some indicators. Second, the evaluation based on an interrupted time series study design estimated relative risk and attributable fraction and assessed VE in the overall population using a multivariable analysis adjusted for sex, age, long-term care grade, disability grade, pregnancy status, cancer registration status, and underlying comorbid conditions, including diabetes, hypertension, kidney disease, lung disease, heart disease, liver disease, and other immune-related diseases. Additionally, a daily dynamic cohort was constructed to control for variability due to time-varying environmental factors, such as temperature and school vacations. As a result, the VE within 2 months after vaccination was 33.4% against hospitalization and outpatient visits, and the VE against hospitalization alone was 41.8%. VE against death was 38.1% which was sustained for 6 months (Supplementary Figure 6; available online). Additionally, vaccination was shown to reduce inpatient and outpatient cases by 143,868, hospitalizations by 12,024, and deaths by 3,506.

Conclusion

This report is significant in that it expanded the scope of infectious disease monitoring by utilizing KDCA surveillance data, NHIS claims data, and their linked dataset. The resulting indicators encompass the entire public health response process, from disease incidence to healthcare utilization, disease burden, and VE evaluation. These comprehensive findings will provide the evidence needed to evaluate influenza epidemic patterns from multiple perspectives and to regularly review and improve the appropriateness of control measures.

In the future, we aim to improve the quality of evidence for influenza management by addressing current limitations and refining the methodologies. To minimize the potential for healthy vaccinee bias and confounding in observational studies along with the lack of detailed subtype information in NHIS claims data, it is necessary to present an analysis using a test-negative design (TND) among patients visiting healthcare institutions who underwent influenza testing. However, as TND analysis may also have limitations in subtype-specific analyses due to factors such as limited sample size, we plan to enhance the influenza VE evaluation system through a multifaceted evaluation of available data, including linked analyses with laboratory surveillance data. Additionally, it would be valuable to extend healthcare cost analyses beyond seasonal aggregates to examine temporal trends in the demand for key healthcare resources, such as emergency departments, general wards, and intensive care units, stratified by influenza epidemic periods. Finally, focusing on adults aged ≥65 years residing in long-term care hospitals and facilities, who account for a substantial proportion of inpatients, will provide essential evidence for establishing strategic infection control policies for high-risk care facilities.

This annual report will be published regularly and made available on the websites of both the KDCA and the NHIS as a resource for healthcare professionals and the general public. The continuous publication and improvement of this report will contribute to strengthening an evidence-based infectious disease control system and minimizing the public health burden of influenza.

Supplementary Materials

Supplementary data are available online.