PHWR Public Health Weekly Report

Objectives: To describe influenza activity in the Republic of Korea during the 2023–2024 and 2024–2025 seasons following the coronavirus disease 2019 (COVID-19) pandemic, using sentinel surveillance data, and to compare season-specific epidemiological patterns.
Methods: Sentinel surveillance data on patients with influenza-like illness (ILI), those hospitalized for influenza, and influenza virological findings reported to the Korea Disease Control and Prevention Agency from week 36 of 2023 to week 35 of 2025 were analyzed.
Results: In the 2023–2024 season, influenza activity returned to pre-COVID-19 levels, with ILI peaking in week 49 of 2023 and increased hospitalizations compared with the previous season. In the 2024–2025 season, ILI reached its highest level since 2016, when a comparable surveillance system was established. In both seasons, school-aged children and adolescents (7–18 years) were the main drivers of influenza transmission, whereas hospitalizations were most common among adults aged ≥65 years. Virological surveillance confirmed the circulation of both influenza A and B viruses.
Conclusions: Post-COVID-19 influenza patterns differed from those previously observed, likely reflecting the continued effects of immunity debt. These findings support strengthening sentinel surveillance, expanding outpatient sites to improve regional representativeness, and providing timely influenza surveillance updates to healthcare professionals and the public through weekly reports and the FluON dashboard.

Objectives: Mycoplasma pneumoniae is a major pathogen responsible for upper and lower respiratory tract infections, accounting for approximately 20–40% of community-acquired pneumonia cases. The infection occurs most frequently in adolescents, with periodic outbreaks occurring every 3–4 years. The present study aimed to characterize the molecular epidemiological features of M. pneumoniae strains isolated during the 2023–2024 epidemic in the Republic of Korea by analyzing macrolide resistance–associated mutations and multilocus sequence typing (MLST) genotypes.
Methods: From January 2023 to December 2024, 1,294 respiratory specimens were collected through the Acute Respiratory Infection Network. Macrolide-resistant mutations in the 23S rRNA domain V (A2063G/A2064G) were analyzed in M. pneumoniae polymerase chain reaction (PCR)-positive specimens. In addition, 217 cultured isolates were further examined for resistant mutations and MLST genotypes.
Results: Among 1,294 collected specimens, 431 (33.3%) were PCR-positive for M. pneumoniae , and 217 (16.8%) isolates were successfully cultured. The A2063G mutation was detected in 81.4% (351/431) of PCR-positive samples and 89.4% (194/217) of cultured isolates. MLST analysis identified four sequence types (STs): ST3 (89.9%) was predominant, followed by ST7 (6.5%), ST14 (1.8%), and ST17 (1.8%). The distribution of STs varied by resistant genotype, with ST3 (97.4%) predominating among resistant isolates and ST7 (52.2%) among susceptible isolates.
Conclusions: M. pneumoniae isolates collected in the Republic of Korea during 2023–2024 exhibited high rates of macrolide resistance (89.4%), with ST3 as the predominant genotype. Distinct MLST distributions were observed between resistant and susceptible strains, underscoring the need for continuous molecular surveillance to monitor periodic outbreaks and shifts in genotype.