PHWR Public Health Weekly Report

Influenza is a major seasonal respiratory illness caused by the influenza virus, typically peaking in incidence during the winter and spreading widely within communities. Owing to these seasonal characteristics, surveillance findings on the influenza virus, such as epidemic timing, detection status, and predominant subtypes, provide the scientific basis for effective policies for infectious disease control, which includes vaccination strategies, public awareness campaigns for epidemic management, and patient care in clinical settings. Accordingly, this report aims to share the domestic epidemic characteristics of the influenza virus during the 2024–2025 season, as observed through pathogen surveillance.

According to the results of sentinel pathogen surveillance conducted by the Korea Disease Control and Prevention Agency (KDCA), the domestic epidemic patterns of influenza and other respiratory viruses in recent years have deviated from their typical seasonality, showing different trends each season [1-3]. Since 2022, the influenza virus has deviated from its usual winter epidemic pattern, with circulation continuing into the summer months, and has been observed to reach progressively higher peaks with each passing year. The coronavirus disease 2019 (COVID-19) pandemic can be considered a reason for this shift. The pandemic, which began in early 2020, led to the implementation of high-intensity non-pharmaceutical interventions such as social distancing and restrictions on gatherings. These measures effectively suppressed the transmission of severe acute respiratory syndrome coronavirus 2 and simultaneously resulted in a reduction in the detection of various respiratory viruses, including the influenza virus, to unprecedentedly low levels compared to those in the past. In fact, influenza was scarcely detected in the Republic of Korea until July 2022. It began to be redetected in August 2022, with epidemics resuming from the 2022–2023 season. In the 2022–2023 season, the A(H3N2) subtype was predominant, marking the first full-scale influenza epidemic since the start of the COVID-19 pandemic. In the 2023–2024 season, a relatively high level of detection was observed from the beginning of the season compared with the previous one. A(H1N1)pdm09 was the dominant strain initially; however, as the peak period (weeks 48–50) approached, the detection of influenza B virus gradually increased, leading to a co-circulation of A and B types. In the 2024–2025 season, the onset of a significant increase in detections was somewhat delayed compared to that in the previous season, with the peak occurring in the first week of 2025. Thereafter, the detection rate surged, reaching a level 20–30% higher than the peak of the previous season (Figure 1). Furthermore, a second wave was observed centered around the school-aged population following the start of the new school semester after the winter break, and an increase in influenza B virus detection was confirmed, rather than that of A(H1N1)pdm09, which had been prevalent early in the season. Unlike the same period in the 2023–2024 season, when detections of all subtypes decreased and were found in similar proportions, the 2024–2025 season was characterized by a predominant increase in influenza B. This increase continued for more than 6 weeks in a gradual manner, driving the second wave and maintaining a high level of detection compared to the same period of the previous season. This suggests that while the seasonal pattern of influenza is gradually returning after the pandemic, it is accompanied by persistence and variability different from those in the past. In addition, unique characteristics were identified in the age-specific detection patterns. In the 2022–2023 and 2023–2024 seasons, the highest detection rates were observed in the school-aged population (7–18 years), suggesting this was primarily influenced by school-centered transmission. However, in the 2024–2025 season, epidemics were identified in age groups other than the school-aged population. During the first wave, between week 50 of 2024 and week 6 of 2025, the detection rate among adults in their 50s and the elderly aged 65 years and above increased relatively, reaching a level similar to that in the school-aged population. The detection rate in the elderly during this period increased by approximately 20% compared to that in the same period in the previous year. During the second wave, when the influenza B epidemic gained momentum, the school-aged population was once again emerged as the primary driver of the epidemic (Figure 2). This epidemic among older adults is presumed to be an expanded effect of the immunity gap formed by reduced opportunities for acquiring natural immunity during the pandemic. This finding is consistent with reports suggesting that vaccine-induced immunogenicity can also result in diminished immune defense if natural exposure is limited, thereby increasing susceptibility to infection [4,5]. Such epidemic patterns have also been reported in some other countries. In the 2023–2024 season, Japan, the United States, and Australia observed influenza epidemic patterns that differed from those seen before the pandemic. In Japan, following an A(H1N1)pdm09 epidemic, the proportion of influenza B cases surged from January 2024, exceeding 60% in March, and the detection rate of influenza B among the elderly increased 1.5-fold [6]. In the United States, types A and B co-circulated, with influenza B persisting until March–May, and a notable increase in infections was observed among the elderly and school-aged children [7]. In Australia, influenza B also surged after April, accounting for 35% of all influenza cases, with high detection rates in adolescents and the elderly [8]. In all three countries, influenza B drove the latter part of the season’s epidemic, and an increase in detections among the elderly was a common feature. This trend has been attributed to an altered immune landscape resulting from the pandemic [9].

Figure 1. Influenza detection rate during 2021-2022–2024-2025 week 17 for review

Figure 2. Distribution of influenza detection rate by age group, 2021-2022–2024-2025 week 17

As discussed in the main body, the domestic influenza pattern during the 2024–2025 season has shown multifaceted changes, similar to the international situation, including delays in epidemic timing, shifts in subtype distribution, and differences in age-specific infection patterns. In particular, the sustained circulation of influenza B virus and the increase in infections among the elderly are the altered epidemic patterns for this season. This underscores the need for future epidemic forecasting and response to be based on effective strategies tailored to evolving trends through close surveillance, rather than relying on past seasonal patterns. Furthermore, it suggests that pathogen surveillance has become even more critical in formulating public health policies. In response, the KDCA is closely monitoring pathogen and patient epidemic situations through both pathogen and clinical surveillance and is strengthening the joint response system for influenza and other respiratory infectious diseases by operating an inter-agency task force for respiratory diseases when necessary. From a preventive perspective, the KDCA continues to provide free influenza vaccinations to high-risk populations as part of the National Immunization Program and is actively promoting vaccination campaigns to improve uptake. Going forward, the KDCA will continue to identify community transmission patterns through close surveillance of respiratory pathogens, including influenza, which circulate throughout the year and will maintain inter-agency cooperation for a swift response.

Ethics Statement: Ethics approval for the study protocol and analysis of the data was obtained from the Institutional Review Board of the Korea Disease Control and Prevention Agency (KDCA; 2022-02-05-C-A).

Funding Source: This study was supported by intramural funds (grant No. 6300-6332-304) from the KDCA.

Acknowledgments: We would like to thank 18 Public Health and Environment Research Institutes (PHERIs).

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: NJL. Data curation: NJL. Formal analysis: SHW, JHL. Investigation: NJL, SHW, JHL, JER, EJK. Methodology: NJL. Project administration: NJL. Resources: NJL, SHW, JHL, JER, EJK. Software: NJL. Supervision: JER, EJK. Visualization: SHW, NJL, JHL. Writing–original draft: NJL. Writing–review & editing: JER, EJK.