Recent infections and transmissions of highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b viruses in humans and animals pose a significant global public health threat. Vaccination is the most effective method for preventing and controlling human infections; however, it is unclear whether the World Health Organization (WHO) candidate vaccine viruses (CVVs) can induce cross-neutralizing antibodies against H5N1 strains currently circulating in the Republic of Korea. In this study, we evaluated sera from ferrets immunized with four WHO HPAI H5 CVVs (clade 1 and clade 2.3.4.4b) against HPAI H5N1 clade 2.3.4.4b viruses isolated from poultry and mammalian outbreaks in the Republic of Korea during 2023–2024. Hemagglutination inhibition and microneutralization assays showed that ferret antisera raised against the H5N1 clade 2.3.4.4b CVVs exhibited broad cross-neutralization against most Korean isolates. Among these, antiserum derived from the A/Ezo red fox/Hokkaido/1/2022 showed the strongest cross-activity. However, ferret antisera raised against the A/Vietnam/1194/2004-derived clade 1 CVV showed negligible neutralizing responses. These findings suggest that H5N1 clade 2.3.4.4b CVVs are effective vaccine candidates for inducing cross-neutralizing antibodies and could play a critical role in preparedness efforts against potential human infections in the Republic of Korea.

Influenza viruses are segmented negative-sense RNA viruses with high genetic variability, causing seasonal epidemics and pandemics. Whole-genome sequencing from clinical specimens is often hindered by low viral loads and host background, necessitating enrichment-based next-generation sequencing (NGS). In this study, we compared amplicon- and probe capture-based NGS using 26 influenza-positive clinical specimens representing A(H1N1), A(H3N2), and B/Victoria lineages, stratified by cycle threshold (Ct) (<25, 25-30, ≥30). Sequencing performance was assessed based on mapped read proportion, mean coverage depth, and segment-level genome recovery across the eight influenza gene segments. For specimens with Ct <25, amplicon-based NGS showed higher mapped read proportions, while both methods achieved near-complete genome recovery. In the 25 ≤ Ct <30 group, overall genome recovery was comparable between approaches, although subtype-specific differences were observed. For Ct ≥30 specimens, probe capture-based NGS demonstrated significantly higher mapped read proportions, greater mean coverage depth, and near-complete segment recovery for influenza A viruses, whereas amplicon-based NGS showed reduced recovery with frequent segment dropouts. Our findings provide a systematic comparison of enrichment strategies under standardized conditions and offer evidence to inform methodological selection for influenza whole-genome sequencing across diverse clinical viral load ranges.

Objectives:This study characterizes the detection patterns of respiratory viruses and the virological characteristics of influenza viruses in the Republic of Korea from week 36 of 2024 to week 35 of 2025, thereby contributing to evidence-based infection control policies and strengthening public health responses. Methods:Respiratory specimens were collected from patients with influenza-like illness at sentinel medical institutions and tested at 18 regional Institutes of Health and Environment. Molecular assays were conducted to detect respiratory viruses, and the distribution of etiologic agents was subsequently analyzed. The influenza viruses were further characterized through genetic, antigenic, and antiviral-resistance analyses to assess their similarity to the vaccine strains and susceptibility to antiviral agents. Results:Among specimens collected during the 2024-2025 season, influenza virus was the most frequently detected respiratory virus (15.2%), followed by rhinovirus (15.0%), severe acute respiratory syndrome coronavirus 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%). The influenza virus subtypes included A(H1N1)pdm09 (39.6%), influenza B virus (36.8%), and A(H3N2) (23.6%). Circulating influenza viruses were genetically and antigenically similar to the 2024-2025 seasonal vaccine strains, and no antiviral-resistance mutations were detected. Conclusions:The epidemiology of influenza and other respiratory viruses varies across seasons, underscoring the importance of continuous surveillance to inform infection control policies and strengthen public health preparedness. Through the Korea Respiratory Virus Integrated Surveillance System, our division will continue to monitor the circulation of respiratory viruses and the characteristics of influenza viruses and disseminate these findings on an ongoing basis.

The continuous emergence of novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants continues to influence the global coronavirus 2019 (COVID-19) pandemic. Even vaccinated individuals or those with prior infections may experience reinfection, depending on the immune evasion capacity of the circulating variants. Therefore, analyses of variant characteristics and immune escape are critical for informing public health policy. However, as COVID-19 has been downgraded in terms of infectious disease classification, the availability of patient sera has become increasingly limited, restricting the timely immunological analyses of emerging variants. To address this challenge, we established an antiserum production and neutralization assay system using a hamster model and demonstrated the utility of this animal serum-based approach. In this study, antisera against 10 circulating SARS-CoV-2 variants were generated in golden Syrian hamsters, and they showed high titer of humoral and neutralizing antibody using enzyme-linked immunosorbent assay and plaque reduction neutralization test (PRNT) assay. In addition, we compared serum from an unvaccinated patient (n = 1) infected with the BA.5 variant with serum from a BA.5-infected hamster (n = 1). Although the number of comparable samples was limited, the PRNT50 titer patterns in serum from an unvaccinated individual infected with BA.5 and in hamster antisera were similar. These findings indicate that antisera generated in the hamster model can provide timely immunogenicity assessments of newly emerging SARS-CoV-2 variants, thereby contributing to the rapid characterization of variant immune escape and the generation of essential data for future public health preparedness.

Severe acute respiratory infections (SARIs) are a critical public health concern due to their substantial morbidity and mortality rates across age groups. The objective of this study was to investigate the epidemiological characteristics, pathogen distribution, and clinical outcomes of SARIs in South Korea by analyzing nationwide surveillance data. We conducted a prospective, multicenter surveillance study from January 2017 to January 2023 across sentinel hospitals nationwide, coordinated by the Korea Disease Control and Prevention Agency (KDCA). Respiratory specimens were collected and tested for a panel of viral and bacterial pathogens. Clinical data and outcomes were analyzed according to pathogen detection status. A total of 47,857 hospitalized patients with SARIs were enrolled based on standardized criteria: fever (≥ 38 °C), cough, and symptom onset within 10 days prior to hospitalization. Pathogens were identified in 52.5

Objectives:The study aim was to examine the detection patterns of respiratory virus circulation in the Republic of Korea from week 36 of 2023 to week 35 of 2024, with a focus on characterizing influenza viruses. The purpose of this study was to support the selection of vaccine strains and strengthen public health preparedness. Methods:Respiratory specimens were collected from 18,040 symptomatic individuals using a national sentinel surveillance network. Real-time reverse transcription polymerase chain reaction testing was performed to identify the causative respiratory pathogens. For influenza viruses, further analyses were performed to assess genetic similarity with the vaccine strains, antigenic characteristics, and antiviral resistance. Results:Influenza viruses were detected in 15.1% of specimens, with the following subtype distributions: A(H1N1)pdm09 (41.7%), A(H3N2) (29.3%), and B type (29.1%). The genetic analysis showed that the circulating strains belonged to phylogenetic groups similar to the current vaccine strains. Further, no mutations associated with resistance to antiviral drugs (Oseltamivir, Zanamivir, and Peramivir, Baloxavir) were identified. The antigenic analysis confirmed effective neutralizing activity against the vaccine strains. Among respiratory viruses, severe acute respiratory syndrome coronavirus 2 was the most frequently detected (15.1%), followed by rhinovirus (14.9%), adenovirus (9.9%), parainfluenza virus (5.6%), respiratory syncytial virus (5.0%), human metapneumovirus (4.2%), human coronavirus (3.8%), and bocavirus (3.2%). Conclusions:The continuous surveillance of influenza and respiratory virus trends is essential to inform vaccine strain selection and enhance public health response strategies. Our division will conduct continuous surveillance of the epidemiological trends of respiratory viruses, including influenza, and ensure the timely provision of data for public health interventions.

Objectives:Continuous genetic variation in pathogens enhances their infectious potential and promotes the emergence of infectious disease outbreaks, highlighting the need for diagnostic technologies capable of broad-range detection. Herein, we introduce pan-polymerase chain reaction (pan-PCR) and multiplex PCR assays to identify the causative agents of emerging or unknown infectious diseases. Methods:To introduce the research, development, and practical applications of pan-PCR and multiplex PCR assays for pathogen diagnosis, a comprehensive review was conducted. The review focused on recent domestic and international institutional reports and academic literature on public health and PCR-based diagnostic methods. Literature published since the coronavirus disease 2019 pandemic was included. Results:Both technologies have been recognized as core diagnostic approaches to effectively respond to emerging and unknown infectious diseases. Pan-PCR uses conserved gene regions for the initial screening of unknown pathogens, whereas multiplex PCR is used to simultaneously identify specific pathogens, including co-infection cases. These two technologies could be utilized complementarily to identify the causative agents of emerging infectious diseases. Conclusions:Pan-PCR and multiplex PCR show promise as key diagnostic platforms to facilitate proactive responses in the face of infectious disease threats in the future. The simultaneous use of both technologies, capitalizing on their respective strengths in versatility and specificity, is likely to improve diagnostic capabilities for emerging or unknown infectious diseases and strengthen public health surveillance.

BACKGROUND:The COVID-19 pandemic and the associated non-pharmaceutical interventions (NPIs) have significantly altered the circulation of respiratory viruses worldwide. This study aimed to describe the temporal and epidemiological changes in the circulation of major respiratory viruses in the Republic of Korea. METHODS:Data were collected from the Korea Respiratory viruses Integrated Surveillance System (K-RISS) over ten consecutive respiratory seasons (2015-2016-2024-2025). Respiratory specimens (n = 119,657) were obtained from 106 sentinel network clinics. Real-time PCR was used to detect eight viruses: influenza virus (IFV), respiratory syncytial virus (RSV), parainfluenza virus (PIV), human metapneumovirus (HMPV), human coronavirus (HCoV), adenovirus (AdV), human bocavirus (HBoV), and human rhinovirus (HRV). Detection patterns were compared across pre-pandemic (2015-early 2020), pandemic (2020-2023), and post-pandemic (late 2023-mid 2025) periods. RESULTS:During the NPI periods, enveloped viruses (IFV, RSV, PIV, HCoV, and HMPV) nearly disappeared, but resurged with atypical seasonality in 2021-2022 and 2022-2023. RSV and PIV reemerged earlier and at higher detection levels, particularly among children aged 0-6 years. In contrast, non-enveloped viruses (HBoV, HRV, and AdV) remained detectable throughout the pandemic. Overall, viral detection dropped to 44.8 % in the 2020-2021 season but recovered to 57.5 % in 2022-2023. By 2024-2025, most viruses had returned to their pre-pandemic seasonal patterns. CONCLUSION:COVID-19-related NPIs caused profound, virus-specific shifts in respiratory virus circulation. Surveillance through the K-RISS allowed for early detection of re-emergent viruses and guided public health responses. Continuous, multipathogen monitoring is essential for preparing against future disruptions driven by pandemics or environmental changes.

Objectives:This study aimed to assess the prevalence and serotype distribution of adenovirus in the Republic of Korea (ROK) by analyzing detection rates and identifying the predominant circulating serotypes. Methods:Through the Korea Respiratory viruses Integrated Surveillance System, real-time polymerase chain reaction testing was performed on 33,688 respiratory specimens collected from symptomatic individuals in ROK between January 2023 and December 2024. Detection rates of adenovirus were analyzed, and selected positive samples-stratified by age group-were further examined via nucleotide sequencing to determine serotype distribution. Results:The average adenovirus detection rates in 2023 and 2024 were 14.8% and 5.8%, respectively. A significant increase in detection rates was observed from summer to autumn in 2023, peaking at 42.4% during weeks 29 to 33, followed by a gradual decline. By 2024, detection rates decreased and remained below 10%. Age-stratified analysis revealed a higher detection rate among individuals aged 0-12 years, with the highest rate (27.5%) observed in the 0-6 years subgroup. Serotyping revealed serotype 3 as the most prevalent, accounting for 36.0% of cases, followed by serotypes 2, 1, and other subtypes. Significantly, serotype 3 predominated during the summer and autumn of 2023. Additionally, serotype 14, previously unreported in the community, was consistently detected throughout 2024. Conclusions:This analysis suggests that the unusual increase in adenovirus detection during the summer of 2023 may have been driven by an outbreak of serotype 3 and highlights the importance for close monitoring for potential clinical changes following the identification of a new serotype, type 14. Accordingly, the national surveillance system will continue to operate to enhance our understanding of respiratory infectious disease trends and to support effective responses, providing data on pathogen detection patterns and characteristic analyses, including serotype identification.

Objectives:This study aimed to analyze the epidemiological characteristics of influenza virus circulation during the 2024-2025 season in the Republic of Korea (ROK), with a focus on recent trends in detection rates and subtype distribution. Methods:Weekly detection rates, subtype distribution, and age-specific patterns of influenza virus were analyzed using surveillance data from week 36 of 2024 to week 17 of 2025 as reported by the Korea Respiratory Virus Integrated Surveillance System. Results:ROK's 2024-2025 influenza season exhibited two distinct epidemic waves. The first wave began in late 2024 and peaked in week 1 of 2025, with a detection rate of 62.9%, representing a 20-30% increase compared to previous seasons. During this period, a notable increase in detection was observed among individuals over age 50, with influenza A viruses predominating. The second wave emerged in March 2025 and was driven by an increase in influenza B virus detection, which accounted for more than 50% of all influenza cases. The circulation of B virus was particularly prominent among school-aged children (7-18 years) and lasted longer than previous seasons. Conclusions:The 2024-2025 influenza season was characterized by increased detection levels compared with prior seasons, prolonged circulation of influenza B viruses, and a marked rise in infections among older adults. These findings suggest an ongoing shift in population immunity following the coronavirus disease 2019 pandemic. To respond effectively to the evolving patterns of respiratory virus circulation, the Korea Disease Control and Prevention Agency will continue to strengthen its timely detection and response efforts through enhanced surveillance systems.

Since the onset of the COVID-19 pandemic, the Republic of Korea has experienced continuous waves of SARS-CoV-2 variants. The current study aimed to analyze the long-term trends of variant prevalence and associated changes in immune responses within the country. Whole-genome sequencing was performed on confirmed patient samples collected from December 2020 to May 2025, and variant distribution, genetic diversity, and neutralization were compared. As a result of analyzing a total of 157,962 gene sequences, various Omicron sub-lineages, including BA.1, BA.2, BA.5, followed by JN.1, KP.3, and NB.1.8.1, were seen to circulate sequentially over time. The nucleotide diversity of the SARS-CoV-2 genome gradually increased after the JN.1 outbreak. Of the tested variants, hamster antiserum neutralization analysis indicated that Omicron NB.1.8.1, which began to circulate in 2025, exhibited the lowest neutralization activity, with an approximately 6.6-fold decrease compared to JN.1. This suggests a potential expansion in the dominance of new variants with enhanced immune evasion. As the transmission of SARS-CoV-2 continues, new variants with novel characteristics may emerge; therefore, continuous national genomic surveillance and immunological characterization are considered crucial for early detection of emerging variants and for guiding effective public health responses.

OBJECTIVES:The etiologic pathogen is unknown for many pediatric community-acquired pneumonia (CAP) cases. We aimed to identify the causes of CAP of unknown etiology (CAP-UKN) using broad-panel targeted next-generation sequencing (tNGS). METHODS:A prospective surveillance study was conducted across 26 hospitals in Korea (September 2023 to November 2024). CAP cases with no identified pathogen were defined as CAP-NPD; cases wherein no pathogen or only human rhinovirus (HRV), human bocavirus (HBoV), human coronavirus (HCoV), or normal colonizing bacteria were detected were classified as CAP-UKN. Residual respiratory specimens were analyzed using 16S rRNA sequencing and tNGS. RESULTS:Among 605 pediatric CAP cases, 178 (29.4%) had CAP-UKN, including 77 CAP-NPD. CAP-NPD was more common at ages 5-10 years with clinical features similar to Mycoplasma pneumoniae pneumonia. HRV/HBoV/HCoV-positive cases resembled those of viral pneumonia. 16S rRNA sequencing and tNGS identified additional pathogens in 23.8% and 70.8% of CAP-UKN specimens, respectively: Haemophilus influenzae, Moraxella catarrhalis, and viridans streptococci (6.3% each) by 16S rRNA sequencing, and Streptococcus pneumoniae (45.5%) and betaherpesvirus (5.2%) by tNGS. CONCLUSIONS:Pediatric CAP-UKN may be associated with undetected or atypical pathogens. HRV, HCoV, or HBoV infections may contribute to some pediatric CAP cases in which no other pathogen is detected.

The ongoing COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to the emergency of various lineages through mutations and recombination. In the Delta lineage, we identified recombination events in the ORF1a gene, which divided the Delta sublineages into three different genotypes (Delta R1-R3). The regional distributions of Delta R1 and Delta R2 were not correlated, indicating that recombination occurred early in the Delta outbreak. The impact of the ORF1a gene on SARS-CoV-2 transmission remains unclear; however, our findings suggest that recombination may have contributed to the evolution and global spread of the Delta lineage.

The adenovirus detection rate is <10% throughout the year in South Korea; however, during the summer of 2023, it showed an unusual increase. We analyzed the adenovirus detection rate using data from the Korea Respiratory Integrated Surveillance System before and after coronavirus disease (COVID-19) collected from 2019 to week 36 of 2023. Before the COVID-19 outbreak in 2019, the mean detection rate was 8.2%, which decreased to 6.1% during the COVID-19 pandemic from 2020 to 2022. In 2023, the mean detection rate was 14.3% in week 36 and the highest in week 34, at 42.2%, and adenovirus was predominantly detected in the summer. The detection rate by age group showed substantially high activity among 0-12-yr-olds after the pandemic. This age group had a steady mean rate of 9.5% during the pandemic, without seasonality. In 2023, the detection rate surged in the 0-6-yr and 7-12-yr age groups, peaking at 61.6% and 57.1%, respectively. The dominant epidemic serotypes were HAdV-1 and HAdV-2 during and HAdV-3 after the pandemic. The multifaceted non-pharmaceutical interventions during the COVID-19 pandemic considerably impacted the prevalence of common respiratory viruses and complicated respiratory virus patterns after the pandemic. Constant surveillance is crucial for epidemic preparedness to monitor the possible surge of certain respiratory viruses.

The prevalence of highly pathogenic avian influenza (HPAI) A(H5N1) viruses has increased in wild birds and poultry worldwide, and concomitant outbreaks in mammals have occurred. During 2023, outbreaks of HPAI H5N1 virus infections were reported in cats in South Korea. The H5N1 clade 2.3.4.4b viruses isolated from 2 cats harbored mutations in the polymerase basic protein 2 gene encoding single amino acid substitutions E627K or D701N, which are associated with virus adaptation in mammals. Hence, we analyzed the pathogenicity and transmission of the cat-derived H5N1 viruses in other mammals. Both isolates caused fatal infections in mice and ferrets. We observed contact infections between ferrets, confirming the viruses had high pathogenicity and transmission in mammals. Most HPAI H5N1 virus infections in humans have occurred through direct contact with poultry or a contaminated environment. Therefore, One Health surveillance of mammals, wild birds, and poultry is needed to prevent potential zoonotic threats.

Objectives: We analyzed the correlation between the infectivity and transmissibility of the severe acute respiratory syndrome coronavirus 2 Omicron sublineages BA.1, BA. 2, BA.4, and BA.5.Methods: We assessed viral replication kinetics and infectivity at the cellular level. Nasopharyngeal and oropharyngeal specimens were obtained from patients with coronavirus disease 2019, confirmed using whole-genome sequencing to be caused by the Omicron sublineages BA.1, BA.2, BA.4, or BA.5. These specimens were used to infect Vero E6 cells, derived from monkey kidneys, for the purpose of viral isolation. Viral stocks were then passaged in Vero E6 cells at a multiplicity of infection of 0.01, and culture supernatants were harvested at 12-hour intervals for 72 hours. To evaluate viral replication kinetics, we determined the cycle threshold values of the supernatants using real-time reverse transcription polymerase chain reaction and converted these values to genome copy numbers.Results: The viral load was comparable between BA.2, BA.4, and BA.5, whereas BA.1 exhibited a lower value. The peak infectious load of BA.4 was approximately 3 times lower than that of BA.2 and BA.5, while the peak load of BA.2 and BA.5 was about 7 times higher than that of BA.1. Notably, BA.1 demonstrated the lowest infectivity over the entire study period.Conclusion: Our results suggest that the global BA.5 wave may have been amplified by the higher viral replication and infectivity of BA.5 compared to other Omicron sublineages. These analyses could support the rapid assessment of the impact of novel variants on case incidence.

During the 2022-2023 season, spanning 36 weeks in 2022 and 35 weeks in 2023, we performed real-time reverse transcription polymerase chain reaction tests on 15,009 respiratory specimens to analyze the causative pathogens and viral characteristics. Of these, 1,341 cases (8.9%) tested positive for influenza. Among positive cases, 1,085 cases (80.9%) were identified as A(H3N2), 211 (15.7%) as A(H1N1)pdm09, and 45 (3.4%) as type B. Genotype analysis confirmed similarity to vaccine strains. Furthermore, antigens from isolated influenza viruses exhibited effective neutralizing activity against vaccine strains and lacked resistance to oseltamivir, zanamivir, and peramivir treatments. Regarding other respiratory viruses, Rhinovirus was the most prevalent, detected in 1,978 cases (13.2%), followed by adenovirus (1,564 cases, 10.4%), metapneumovirus (1,456 cases, 9.7%), parainfluenza virus (1,430 cases, 9.5%), respiratory syncytial virus (1,139 cases, 7.6%), bocavirus (794 cases, 5.3%), and human coronavirus (742 cases, 4.9%). Following the relaxation of coronavirus disease 2019 control measurements, we observed a seasonal increase in respiratory viral diseases, highlighting the importance of national respiratory viral surveillance. Our department remains committed to closely monitoring causative pathogens and analyzing influenza virus trends and characteristics.

BACKGROUND:The coronavirus disease 2019 (COVID-19) pandemic led to a decrease in the seasonal incidence of many respiratory viruses worldwide due to the impact of nonpharmaceutical interventions (NPIs). However, as NPI measures were relaxed, respiratory viral infections re-emerged. We aimed to characterize the epidemiology of respiratory viruses in Korean children during post-COVID-19 pandemic years compared to that before the pandemic. METHODS:A nationwide prospective ongoing surveillance study has been conducted for detection of respiratory viruses between January 2017 and June 2023. We included data on adenovirus (AdV), human bocavirus (HBoV), human coronavirus (HCoV), human metapneumovirus (HMPV), human rhinovirus (HRV), influenza virus (IFV), parainfluenza virus (PIV), and respiratory syncytial virus (RSV), which were detected in children and adolescents younger than 20 years. We analyzed the weekly detection frequency of individual viruses and the age distribution of the affected children. The study period was divided into prepandemic (2017-2019) and postpandemic (2021-2023) periods. RESULTS:A total of 19,589 and 14,068 samples were collected in the pre- and postpandemic periods, respectively. The overall detection rate of any virus throughout the study period was 63.1%, with the lowest occurring in the 2nd half of 2020 (50.6%) and the highest occurring in the 2nd half of 2021 (72.3%). Enveloped viruses (HCoV, HMPV, IFV, PIV, and RSV) almost disappeared, but nonenveloped viruses (AdV, HBoV, and HRV) were detected even during the peak of the COVID-19 pandemic. The codetection rate increased from 15.0% prepandemic to 19.1% postpandemic (P < 0.001). During the postpandemic period, a large out-of-season PIV and HMPV epidemic occurred, but the usual seasonality began to be restored in 2023. The mean age of children with each virus detected in 2023 was significantly greater than that in prepandemic years (P = 0.003 and 0.007 for AdV and HCoV, respectively; P < 0.001 for others). The mean age of children with IFV increased in 2022 (11.1 ± 5.2 years) from prepandemic years (7.9 ± 4.6 years) but decreased to 8.7 ± 4.1 years in 2023. CONCLUSION:With the relaxation of NPI measures, several seasonal respiratory viruses cocirculated with unusual seasonal epidemic patterns and were associated with increasing age of infected children.

Since the outbreak of the coronavirus disease (COVID-19) pandemic in 2019, the Korea Disease Control and Prevention Agency (KDCA) has been conducting genomic surveillance using whole-genome sequencing to monitor SARS-CoV-2 variants in the Republic of Korea (ROK). Since the JN.1 lineage, derived from BA.2.86, was first detected in ROK in November 2023, the proportion of various JN.1 sub-lineages has continued to increase, with KP.3 accounting for 60.9% as of August 2024. In particular, the KP.3 sub-lineages with the highest shares were identified as KP.3.3.1 (22.3%), KP.3.3 (14.0%), and KP.3.1.1 (11.1%). We compared cell-based infectivity and viral replication among recently circulating JN.1, KP.2, and KP.3. KP.3 showed the highest viral shedding rate up to 48 hours, especially at 24 hours, when it was approximately 67 times higher than that of JN.1 and 23 times higher than that of KP.2. Although there was little difference in peak viral replication among JN.1, KP.2, and KP.3, that of KP.3 was approximately 66 and 16 times higher than that of JN.1 and KP.2, respectively, at 24 hours. We hypothesize that the higher initial infectious virus shedding and viral replication of KP.3 compared to those of JN.1 and KP.2 may have contributed to the increase in transmission and cases. The KDCA will continue close monitoring based on whole-genome sequencing to identify the prevalence of variants in ROK, characterize new variants, and provide scientific evidence to guide the COVID-19 response in the country.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (COVID-19), is co-infection with other respiratory viruses can negatively affect the patient's clinical condition and cause difficulty in breathing, comorbidities, and increased mortality risk. Hence, the Korea Centers for Disease Control and Prevention Agency analyzed the co-infection rate of respiratory viruses, such as influenza, and the distribution by age groups in domestic COVID-19 patients. Co-infection with one or more respiratory viruses was confirmed in 104 (2.0%) of 5,171 COVID-19-positive samples collected from January to December 2022. The detection rate of co-infection with respiratory pathogens was highest at 15.8% in the age group of 0-6 years. The immature immune system of children and environmental resistance of non-enveloped viruses may have contributed to the high co-infection detection rate. As the spread of respiratory viruses is increasing due to relaxation of the COVID-19 restrictions and co-infections are expected to increase, we will continue to monitor co-infections of SARS-CoV-2 and other respiratory viruses closely.