PHWR Public Health Weekly Report

Influenza is an acute respiratory disease caused by the influenza virus and is commonly known as the flu. It is an infectious disease estimated to infect approximately 1 billion people worldwide each year, of whom 3–5 million develop severe illness and 290,000–650,000 die [1]. The World Health Organization (WHO) recommends that each country operate a systematic and continuous influenza surveillance system. In the Republic of Korea (ROK), influenza has been designated as a Class 4 notifiable infectious disease under the “Infectious Disease Prevention and Control Act (hereafter, the Infectious Disease Control Act),” and a sentinel surveillance system is currently in operation [2].

In 1997, the ROK initiated a pilot sentinel surveillance system involving approximately 70 private medical institutions nationwide. In September 2000, a national surveillance system for “influenza-like illness (ILI)” was established, involving primary care clinics. During the 2009 influenza A(H1N1) pandemic, the number of sentinel surveillance sites was expanded to 814, approximately one site per 50,000 population. For more efficient operation of the surveillance system, the number of sites was reduced to 200 in 2013. As the operational capacity of the surveillance system improved, the number of sites was further expanded to 300 in 2024.

Sentinel surveillance sites for ILI consist of primary care clinics in pediatrics, internal medicine, family medicine, and otorhinolaryngology. The surveillance system is operated by defining one influenza season from week 36 of each year to week 35 of the following year. Designated sentinel surveillance sites report the weekly number of ILI cases and the total number of outpatient visits through the Integrated Disease and Health Control System (https://eid.kdca.go.kr) or by fax. Among the clinics participating in ILI surveillance, some also participate in respiratory pathogen surveillance (Supplementary Figure 1; available online).

The results of ILI surveillance are used as indicators for establishing epidemic thresholds and determining the occurrence of influenza epidemics. The results of pathogen surveillance provide foundational data for influenza response by identifying circulating viral genotypes during each season and assessing their match with vaccine strains [3,4].

Influenza hospitalization surveillance began in 2015 through the Acute Respiratory Infection Sentinel Surveillance System, which was initially established in 2011 with 87 hospitals (≥300 beds). Beginning in 2017, sentinel surveillance sites were expanded to 206 hospitals with ≥200 beds. During the 2023–2024 season, a total of 220 hospitals participated in influenza hospitalization surveillance.

Surveillance data on ILI, pathogens, and hospitalized influenza cases are disseminated through the “Infectious Disease Sentinel Surveillance Weekly Report” and the integrated online influenza surveillance dashboard (FluON) [5].

This article summarizes the results of influenza sentinel surveillance during the 2023–2024 (week 36 of 2023 to week 35 of 2024) and 2024–2025 seasons (week 36 of 2024 to week 35 of 2025).

We analyzed the results of sentinel surveillance for ILI, influenza viruses, and hospitalized influenza cases during the 2023–2024 and 2024–2025 seasons, from week 36 of 2023 through week 35 of 2025.

For ILI surveillance, approximately 251 primary care clinics in internal medicine, pediatrics, and family medicine participated during the 2023–2024 season. In the 2024–2025 season, otorhinolaryngology clinics were added, and, following the expansion of surveillance sites, approximately 299 clinics participated. The mean performance rates of ILI sentinel surveillance were 99.0% and 99.1% for the 2023–2024 and 2024–2025 seasons, respectively.

ILI was defined as a sudden onset of fever ≥38°C accompanied by cough or sore throat. Using the weekly number of ILI cases and the total number of outpatient visits reported by sentinel primary care clinics, the proportion of ILI cases per 1,000 outpatients was calculated.

For influenza hospitalization surveillance, 220 hospitals with ≥200 beds participated during the 2023–2024 and 2024–2025 seasons. An influenza case was defined, in accordance with the “Notification on Diagnostic Criteria for Reporting Infectious Diseases,” as a patient with clinical symptoms consistent with influenza and detection of influenza-specific genes in clinical specimens (oropharyngeal swab, nasopharyngeal swab, nasopharyngeal aspirate, nasal aspirate, bronchoalveolar lavage fluid, or sputum) [3,4].

Influenza virus surveillance results were calculated as the influenza virus detection rate based on the number of tests performed on the specimens requested weekly by sentinel primary care clinics.

1. Influenza-like Illness Surveillance at Primary Care Clinics

As coronavirus disease 2019 (COVID-19) control measures were being eased, an influenza epidemic advisory was issued in week 37 of 2022 (early September; 2022–2023 season epidemic threshold: 4.9 cases per 1,000 outpatients). As summer transmission continued, the advisory remained in effect without being lifted from the start of the 2023–2024 season. During the 2023–2024 season, the proportion of ILI began to gradually increase from week 37 of 2023 (mid-September) and rose sharply after week 48 (late November), reaching a peak of 61.3 cases per 1,000 outpatients in week 49 of 2023, thereby showing a return to the pre–COVID-19 epidemic pattern. After reaching the peak, ILI activity showed a gradual declining trend without a secondary spring outbreak and remained below the 2024–2025 seasonal epidemic threshold (8.6 cases per 1,000 outpatients) for three consecutive weeks, leading to the lifting of the influenza epidemic advisory in July 2024, which had been maintained for approximately 22 months.

During the 2024–2025 season, the proportion of ILI was lower in October and November compared with the 2023–2024 season, but began to increase sharply in early December, reaching a peak of 99.8 cases per 1,000 outpatients in week 1 of 2025 (early January). This level was the highest observed since 2016, when a sentinel surveillance system at the current level was established (week 52 of 2016: 86.2) (Figure 1).

Figure 1. Influenza-like illness (ILI) rates and warning of influenza over 2022–2023 to 2024–2025 season

By age group, the 7–18-year age group led the epidemic in both seasons. In the 2024–2025 season, a minor spring outbreak occurred following the start of the school term, driven by increased incidence in the school-aged group (Figure 2).

Figure 2. Age-specific influenza-like illness (ILI) rates during the 2023–2024 and 2024–2025 seasons

2. Influenza Hospitalization Surveillance in Hospitals

In the 2023–2024 season, a total of 12,225 hospitalized influenza cases (approximately 55.6 cases per institution) were reported from 220 hospitals with ≥200 beds, representing a 57.6% increase compared with 7,755 cases (approximately 35.3 cases per institution) in the previous season. In the 2024–2025 season, a total of 8,640 hospitalized influenza cases (approximately 39.1 cases per institution) were reported, representing a 29.3% decrease compared with 12,225 cases (approximately 55.6 cases per institution) in the previous season. However, the peak number of hospitalized influenza cases was 1,632 (week 2 of 2025) during the 2024–2025 season, which was approximately 48.2% higher and occurred about one month later than the peak of 1,101 cases (week 50 of 2023) during the 2023–2024 season (Figure 3).

Figure 3. Hospitalized influenza cases over the most recent five seasons

By age group, cases in the 2023–2024 season occurred in the following order: ≥65 years (37.8%), 7–12 years (14.2%), 50–64 years (13.3%), 1–6 years (13.2%), 19–49 years (12.9%), 13–18 years (6.6%), and 0 years (2.0%). In the 2024–2025 season, cases occurred in the following order: ≥65 years (52.4%), 50–64 years (15.3%), 7–12 years (9.2%), 19–49 years (9.1%), 1–6 years (7.4%), 13–18 years (4.2%), and 0 years (2.4%). In both seasons, individuals aged ≥65 years accounted for the highest number of influenza cases. In the 2023–2024 season, those aged ≥65 years accounted for 37.8% of all cases (n=4,619), whereas this proportion increased to 52.4% (n=4,528) in the 2024–2025 season, indicating a higher proportion of cases among individuals aged ≥65 years (Figure 4).

Figure 4. Age-specific hospitalized influenza cases during the 2023–2024 and 2024–2025 seasons

3. Pathogen Surveillance

During the 2023–2024 season, pathogen surveillance showed a peak detection rate of 43.8% in week 50 of 2023. A(H1N1) predominated early in the season, followed by increases in A(H3N2) and B viruses, resulting in the co-circulation of three subtypes, with transmission particularly concentrated in the school-aged group (7–18 years) (Figure 5).

Figure 5. Detection rate of influenza viruses during the 2023–2024 and 2024–2025 seasons

In the 2024–2025 season, the influenza virus detection rate was delayed compared with the previous season, but rose sharply after December 2024, reaching levels 20–30% higher than the previous season’s peak detection rate. There was a first outbreak centered on influenza A viruses, which peaked in week 1 of 2025, and a second outbreak centered on influenza B viruses in March 2025, resulting in two peaks. During the first wave, detection rates increased among older adults aged ≥50 years. During the second wave, influenza B accounted for more than 50% of total detections, with the outbreak centered on the school-aged group (7–18 years), and the duration was longer than the first wave (Supplementary Figure 2; available online).

Taken together, the influenza sentinel surveillance results from the 2023–2024 and 2024–2025 seasons indicate that the occurrence of respiratory infectious diseases, which had been suppressed during the COVID-19 pandemic, showed clear epidemic patterns over the two seasons. During the 2023–2024 season, as influenza activity in the community returned to pre–COVID-19 levels, the proportion of ILI reached a peak of 61.3 in week 49 of 2023, and the number of hospitalized influenza cases increased by 57.6% compared with the previous season. This finding might be attributed to the resumption of respiratory virus circulation in the community after social distancing and other measures implemented during the COVID-19 pandemic were lifted, together with the accumulation of a population susceptible to influenza.

During the 2024–2025 season, the proportion of ILI reached 99.8 cases per 1,000 outpatients (week 1 of 2025), the highest level observed since 2016, when a sentinel surveillance system at a level comparable to the current one became established. In both seasons, the highest incidence of ILI was observed in the 7–18-year group. Particularly after the start of the school term, repeated minor outbreaks in this group led to the epidemic in the community. This finding appears to be related to the co-circulation of three subtypes, with A(H1N1)pdm09 and A(H3N2) predominating initially, followed by an increase in influenza B viruses.

During the relevant seasons, the WHO recommended vaccine virus strains targeting influenza A(H1N1), A(H3N2), and B. The major circulating viruses isolated through the sentinel surveillance system generally showed antigenic characteristics consistent with the WHO-recommended A(H1N1)pdm09, A(H3N2), and B/Victoria lineage-like strains. These findings suggest that the influenza epidemic and occurrence patterns were more likely influenced by high-level circulation of virus lineages included in the recommended vaccine strains, rather than by the emergence of novel variant viruses with antigenic mismatch to the vaccine. In particular, reduced opportunities for immunity acquisition through natural infection during the COVID-19 pandemic led to the accumulation of immunity debt, and this increase in the susceptible population, regardless of vaccination status or coverage, may have acted in combination to expand the scale of the epidemics.

Meanwhile, although the proportion of ILI during the 2024–2025 season was the highest observed since 2016, the number of hospitalized influenza cases was lower than in the previous season. This finding might be attributed to improvements in reporting practices following the clarification of diagnostic testing criteria for influenza rather than being interpreted as a change in influenza severity. Previously, through the influenza hospitalization surveillance system, hospitalized influenza cases were reported based not only on polymerase chain reaction (PCR) testing but also on other diagnostic methods such as rapid antigen tests. However, beginning in the 2024–2025 season, the influenza management guidelines clarified the reporting criteria for hospitalized influenza cases by PCR testing as the diagnostic testing standard for influenza under the “Notification on Diagnostic Criteria for Reporting Infectious Diseases,” and sentinel surveillance sites were notified accordingly. As a result, the reporting system was reorganized to require that only hospitalized cases confirmed by PCR, excluding other diagnostic methods, be reported. Consequently, the number of hospitalized influenza cases may appear lower than in the previous season, and caution is needed when interpreting these findings.

Pathogen surveillance identified influenza A viruses as the predominant epidemic viruses in both seasons. In addition, similar to influenza, the seasonality of other respiratory viruses, such as respiratory syncytial virus, showed a recovery pattern resembling that observed prior to the COVID-19 pandemic. These findings suggest that changes in respiratory virus epidemic patterns following the relaxation of non-pharmaceutical interventions after the COVID-19 pandemic, together with the effects of immunity debt, may continue.

Furthermore, in recent seasons, influenza epidemic peaks have occurred earlier than in previous years or have persisted into late spring, unlike in the past. These changes in the timing of influenza epidemics raise the need to re-evaluate existing vaccination strategies and surveillance operations. In particular, discussion and review are needed regarding the adjustment of vaccination timing or additional vaccination strategies for high-risk groups to ensure that the effectiveness of the vaccine is sufficiently maintained.

This report is based on data reported through the sentinel surveillance system and, therefore, has limitations in directly reflecting the overall level of influenza occurrence nationwide. Interpretation should take into account potential underreporting by some medical institutions and differences in representativeness and accuracy resulting from the clarification of reporting criteria. Nevertheless, the influenza sentinel surveillance results from the 2023–2024 and 2024–2025 seasons provide important evidence for understanding the epidemiological characteristics of influenza in the ROK after the COVID-19 pandemic and for establishing future response strategies.

Influenza has recovered to levels exceeding the pre-COVID-19 incidence and is occurring in patterns different from those observed prior to the pandemic, increasing the importance of operating a more stable and systematic influenza sentinel surveillance system.

To this end, the Korea Disease Control and Prevention Agency plans to expand the number of primary care influenza sentinel surveillance sites to enhance the regional representativeness of influenza sentinel surveillance and to strengthen surveillance systems for emerging and novel respiratory viruses. In addition, by reorganizing the Sentinel Surveillance Weekly Report and developing FluON, a responsive influenza surveillance dashboard, influenza surveillance information will be provided rapidly and transparently to healthcare professionals and the public, thereby continuously strengthening national capacity for influenza response and management.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

Conflict of Interest: Hyungmin Lee is an editorial board member of the journal, but was not involved in the review process of this manuscript. Otherwise, there are no conflicts of interest to declare.

Author Contributions: Conceptualization: YKK, JSS, DKK, HML. Data curation: JAK. Formal analysis: JAK. Writing – original draft: JAK. Writing – review & editing: YKK, JSS, DKK, HML.

Supplementary data are available online.