PHWR Public Health Weekly Report

Acute respiratory infectious diseases constitute a major global public health concern, causing high morbidity and substantial healthcare burden. In particular, the influenza virus is a leading cause of outpatient visits and hospitalizations, and its epidemic scale and severity vary by season, underscoring the importance of systematic and continuous surveillance [1]. The Division of Emerging Infectious Diseases at the Korea Disease Control and Prevention Agency operates the Korea Respiratory Virus Integrated Surveillance System (K-RISS) to identify the etiological agents of circulating respiratory infectious diseases and to contribute to the establishment of prevention and control policies based on pathogen detection patterns. K-RISS compiles results for seven respiratory viruses—including influenza and coronavirus disease 2019 (COVID-19)—from specimens collected at 200 medical institutions nationwide (including 106 sentinel outpatient clinics, 59 secondary and tertiary hospitals, five specialized diagnostic institutions, and 30 long-term care hospitals). These viruses include rhinovirus, adenovirus, human bocavirus, human metapneumovirus, parainfluenza virus, respiratory syncytial virus, and human coronavirus. The system provides information on national circulation patterns and generates evidence to support prevention and control policies and public health responses. In particular, the influenza virus, as a representative seasonal respiratory pathogen, exhibits high genetic variability and antigenic changes, necessitating continuous characterization. Therefore, K-RISS evaluates similarity to vaccine strains through genetic and antigenic analyses of influenza viruses and concurrently monitors antiviral resistance mutations to support vaccine policy and clinical treatment strategies. However, detection patterns of respiratory viruses, subtype distributions of influenza virus, and their genetic and antigenic characteristics vary each season, directly influencing epidemic scale and public health response strategies. Therefore, a comprehensive analysis of respiratory virus circulation and influenza virus characteristics for a specific season is critical for evaluating the effectiveness of the national surveillance system and guiding future response strategies. This report presents the epidemiological patterns of respiratory infectious diseases detected during the 2024–2025 season, along with the genetic, antigenic, and antiviral resistance characteristics of influenza viruses. This study aims to enhance the understanding of seasonal respiratory virus dynamics and contribute to the development of prevention and control policies and strengthened public health responses.

1. Detection of Respiratory Viruses

During the 2024–2025 season, upper respiratory specimens were collected from patients with respiratory disease symptoms at sentinel surveillance medical institutions. The collected specimens were tested for influenza and eight respiratory viruses using real-time reverse transcription polymerase chain reaction at 18 regional Institutes of Health and Environment nationwide. The results of these molecular assays were aggregated by our division to analyze national pathogen detection rates, and surveillance findings were disseminated weekly through the Infectious Disease Sentinel Surveillance Weekly Report.

2. Characterization of Influenza Viruses

RNA extracted from influenza-positive specimens was subjected to next-generation sequencing to obtain full genetic sequences. Hemagglutinin (HA) phylogenetic analysis was performed to assess similarity to vaccine strains, while neuraminidase (NA) and polymerase acidic (PA) gene analyses were conducted to identify antiviral resistance–associated mutations.

For virus isolation, A(H1N1)pdm09 and influenza B-positive specimens were inoculated into Madin–Darby canine kidney (MDCK) cells, whereas A(H3N2)-positive specimens were inoculated into MDCK-SIAT1 cells. Isolated viruses were then analyzed for phenotypic resistance to antiviral agents (oseltamivir, zanamivir, and peramivir) and for antigenic characteristics using neutralization assays with vaccine strain antisera (hemagglutination inhibition assay or focus reduction assay) [2].

1. Detection Status of Influenza Viruses

Among the specimens collected during the 2024–2025 season, the influenza virus was detected in 15.2% of cases. Among influenza-positive specimens, subtype distribution was as follows: A(H1N1)pdm09, 39.6%; influenza B (Victoria lineage), 36.8%; and A(H3N2), 23.6% (Table 1). Influenza A virus activity began to increase from week 48 of 2024 and peaked in week 1 of 2025 at 62.9%, representing a ~20% increase compared with the 2023–2024 season. During periods when influenza A predominated, transmission was observed mainly among school-aged children (7–12 years), as well as among adults aged ≥50 years. A second epidemic wave was observed in the spring, primarily among school-aged individuals (7–18 years), during which the influenza B virus was predominantly detected (Figure 1). During the 2024–2025 season, influenza virus detection by age group was highest among individuals aged 13–18 years (27.3%), followed by those aged 7–12 (23.0%), 19–49 (18.2%), 50–64 (15.9%), ≥65 (10.1%), and 0–6 years (5.6%) (Figure 2).

Figure 1. Weekly influenza virus detection in the Republic of Korea during the 2024–2025 season

Figure 2. Respiratory virus detection in the Republic of Korea by age group during the 2024–2025 season
HAdV=human adenovirus; HBoV=human bocavirus; HRV=human rhinovirus; HPIV=human parainfluenza virus; HRSV=human respiratory syncytial virus; HCoV=human coronavirus; HMPV=human metapneumovirus; SARS-CoV-2=severe acute respiratory syndrome coronavirus 2; IFV=influenza virus.

Table 1 Detection rates of influenza viruses in the Republic of Korea

Season	Number of specimens	Detection rate (%)		Detection rate by subtype (%)
		Total		A(H1N1)pdm09	A(H3N2)	B(Victoria)
2024–2025	17,091	15.2		39.6	23.6	36.8

2. Detection Status of Respiratory Viruses

During the 2024–2025 season, influenza virus was the most frequently detected (15.2%), followed by rhinovirus (15.0%), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (9.7%), parainfluenza virus (6.2%), adenovirus (4.9%), human metapneumovirus (4.8%), human coronavirus (4.7%), respiratory syncytial virus (3.6%), and human bocavirus (3.2%) (Table 2). Rhinovirus showed a consistently high detection rate of over 20% throughout the year, but decreased to below 10% during periods when influenza A virus was prevalent. SARS-CoV-2 detection increased during the summer and peaked at 37.7% in week 35 of 2025 (Figure 3). By age group, the influenza virus was most frequently detected in all age groups except among children aged 0–6 years and adults aged ≥65 years, in whom rhinovirus (23.4%) and SARS-CoV-2 (15.3%) were most frequently detected, respectively (Figure 2).

Figure 3. Weekly respiratory virus detection in the Republic of Korea during the 2024–2025 season
HAdV=human adenovirus; HBoV=human bocavirus; HRV=human rhinovirus; HPIV=human parainfluenza virus; HRSV=human respiratory syncytial virus; HCoV=human coronavirus; HMPV=human metapneumovirus; SARS-CoV-2=severe acute respiratory syndrome coronavirus 2; IFV=influenza virus.

Table 2 Detection rates of respiratory viruses in the Republic of Korea

Season	Detection rate (%)
	Influenza	Rhinovirus	SARS-CoV-2	Parainfluenza virus	Adenovirus	Metapneumovirus	Human coronavirus	Respiratory syncytial virus	Bocavirus
2024–2025	15.2	15.0	9.7	6.2	4.9	4.8	4.7	3.6	3.2

SARS-CoV-2=severe acute respiratory syndrome coronavirus 2..

3. Characteristics of Influenza Viruses in the Republic of Korea

Analysis of HA gene sequences of influenza viruses circulating in the Republic of Korea (ROK) showed that A(H1N1)pdm09 viruses belonged to two subclades: 6B.1A.5a.2a.1 (15.2%), which is similar to the vaccine strain, and 6B.1A.5a.2a (84.8%). All A(H3N2) viruses were classified as 3C.2a1b.2a.2a.3a.1 (100.0%), belonging to the J subclade of the vaccine strain. Influenza B viruses were identified as belonging to the Victoria lineage, specifically the V1A.3a.2 (100.0%) clade, which was identical to the vaccine strain. Upon analysis of NA and PA gene sequences to determine the presence of antiviral resistance mutations, no mutations affecting antiviral resistance were identified. During phenotypic analyses, antiviral agents (oseltamivir, zanamivir, and peramivir) were applied to viruses isolated from specimens, and no resistant viruses were identified. Analysis of the neutralizing capacity of antiserum (provided by the World Health Organization) obtained from vaccinated weasels against isolates in the ROK confirmed that the vaccine antiserum was effective against domestic isolates, with neutralizing titers of >40 for all subtypes (A(H1N1)pdm09 160–640, A(H3N2) and B 80–320) (Table 3).

Table 3 Genetic, antigenic, and antiviral-resistance characteristics of influenza viruses in the Republic of Korea during the 2024–2025 season

Virus	Clade of 2024–2025 season vaccine strain	Korea influenza virus in the 2024–2025 season
		Genotype	Drug resistance	Antibody titera)
A(H1N1)pdm09	6B.1A.5a.2a.1	6B.1A.5a.2a (84.8%)	Sensitive	160–640
		6B.1A.5a.2a.1 (15.2%)
A(H3N2)	3C.2a1b.2a.2a.3a.1	3C.2a1b.2a.2a.3a.1(J) (100.0%)	Sensitive	80–320
B(Victoria)	V1A.3a.2	V1A.3a.2 (100.0%)	Sensitive	80–320

^a)Neutralization capacity is effective when the titer is >40..

This study analyzed the circulation patterns of respiratory viruses and the characteristics of influenza viruses detected in the ROK during the 2024–2025 season. Influenza virus showed the highest detection rate (15.2%), followed by rhinovirus (15.0%) and SARS-CoV-2 (9.7%). Two epidemic waves of influenza were observed. In the first wave, influenza A viruses predominated, with A(H1N1)pdm09 driving the epidemic. Detection rates increased gradually after week 48 of 2024 and peaked at 62.9% in week 1 of 2025. In particular, during the influenza A epidemic, detection rates increased among middle-aged (≥50 years) and older adults (≥65 years), showing patterns similar to those in school-aged individuals (7–18 years). This detection rate was the highest since the COVID-19 pandemic and is presumed to reflect increased susceptibility due to reduced immune protection [3]. Subsequently, a gradual increase in influenza B virus activity led to a second epidemic wave, peaking at 28.8% in week 17 of 2025. Influenza B virus circulation was primarily concentrated among school-aged individuals (7–18 years), suggesting that group settings such as schools and educational institutions play an important role in the transmission of respiratory infections [4]. This pattern differs from the 2023–2024 season, during which three influenza subtypes circulated simultaneously. These findings indicate that influenza epidemics should not be predicted solely based on past patterns; rather, close monitoring is necessary to inform timely and appropriate prevention and control policies, highlighting the importance of respiratory pathogen surveillance.

Rhinovirus, although detected year-round, showed decreased detection during periods of influenza A virus circulation. These surveillance results support experimental findings that rhinovirus infection may stimulate antiviral defenses that suppress influenza A virus infection, suggesting potential implications for predicting transmission patterns and designing prevention strategies [5]. SARS-CoV-2 exhibited a seasonal pattern similar to the previous season, with increased circulation beginning in summer and peaking in autumn.

Analysis of whole-genome sequences obtained from influenza virus specimens to assess similarity to vaccine strains and the presence of antiviral resistance mutations showed that influenza viruses circulating in the ROK had high similarity to the 2024–2025 seasonal vaccine strains, and no antiviral resistance mutations were identified. In addition, when antiviral agents were applied to influenza isolates to assess phenotypic resistance, no resistant viruses were identified. These findings indicate that antiviral agents used for the treatment of influenza patients were effective. Furthermore, as antigenic titers were ≥40 for all subtypes, vaccine strain antisera were confirmed to exhibit effective neutralizing activity against circulating isolates, suggesting that influenza vaccination during the 2024–2025 season may have effectively prevented viral infection.

Through surveillance of respiratory pathogens circulating in the ROK, our division will continue to monitor community-level patterns of respiratory infectious diseases and assess the effectiveness of seasonal vaccine strains through influenza virus characterization. By generating genetic and antigenic data to inform vaccine strain selection for subsequent seasons, we will continue to contribute to public health responses and the establishment of prevention and control policies.

Ethics Statement: Ethics approval for the study protocol and analysis of the data was obtained from the Institutional Review Board of the 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 thank 18 Public Health and Environment Research Institutes for support.

Conflict of Interest: Eun-Jin Kim is an editorial board member of the journal, but was not involved in the review process of this manuscript. Otherwise, there is no conflict of interest to declare.

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