Eighteen-year laboratory-based surveillance of human coronavirus OC43 in a single tertiary hospital in the Republic of Korea: Temporal inflection, seasonal stability, and age-dependent risk.

Human coronavirus OC43 (HCoV-OC43), a member of the betacoronavirus genus, is a seasonally circulating respiratory virus that persists globally; however, long-term laboratory-based surveillance data remain limited. We analyzed multiplex real-time PCR test results collected at Dankook University Hospital between 2007 and 2024 to comprehensively evaluate long-term annual trends, seasonal structure, and age-specific detection patterns of HCoV-OC43. Based on nasopharyngeal swab specimens, the annual positivity rate of HCoV-OC43 ranged from 0.16% to 3.05%. A marked suppression in detection was observed during the COVID-19 pandemic (2020-2021) compared with the pre-pandemic years. This was followed by a partial recovery in 2022-2023; however, detection declined again in 2024 without fully returning to pre-pandemic levels. Seasonal analysis demonstrated a consistent winter predominance throughout the study, with the highest positivity observed in winter (4.44%) and the lowest in summer (1.16%), despite interannual variability. Age-specific analysis revealed highest positivity rates in early childhood 1-4 years (3.12%) and among infants aged 0 years (2.71%), followed by a progressive decline in detection across school-age children, adolescents, adults, and older adults, indicating a clear age-dependent distribution. These findings provide laboratory-based evidence characterizing the temporal, seasonal, and age-related detection patterns of HCoV-OC43 using long-term diagnostic data from a single institution. This information may serve as a reference for interpreting routine respiratory virus testing results and understanding seasonal diagnostic demand and surveillance strategies in clinical practice.