Resurgence and seasonal patterns of RSV-B during the COVID-19 era: an 18-year retrospective hospital-based study.

Respiratory syncytial virus (RSV) is a leading cause of severe respiratory infections, particularly in infants, older adults, and immunocompromised individuals. In this study, we aimed to characterize the epidemiology of RSV subtype B (RSV-B), which remains relatively understudied compared with subtype A despite its clinical significance. We conducted a retrospective analysis of laboratory-confirmed RSV-B infections over 18 years (2007-2024) at a tertiary hospital in South Korea. The dataset included 23,284 cases analyzed for age distribution, seasonality, sex differences, and the impact of the coronavirus disease 2019 (COVID-19) pandemic. The highest positivity rate was observed in infants under 1 year (12.7%, p < 0.001), followed by ages 1-19 years (6.9%). RSV-B incidence peaked in winter (11.9%) and autumn (8.7%), with significant seasonal variation (p < 0.001). No statistically significant sex-based difference was observed (male: 6.1%, female: 6.7%; p = 0.102). Positivity rates declined markedly during the COVID-19 pandemic (2019-2022), likely due to non-pharmaceutical interventions. These findings clarify RSV-B's distinct epidemiology and underscore the need for subtype-specific surveillance, targeted vaccination, and adaptable public health strategies. This study provides evidence to improve outbreak prediction, identify high-risk groups, and optimize clinical and preventive responses to RSV-B.