Post-Pandemic Resurgence of Respiratory Syncytial Virus in Thailand: Molecular Epidemiology and Sublineage Turnover Among Children, 2023-2025.

Non-pharmaceutical interventions implemented during the COVID-19 pandemic profoundly disrupted the established seasonality of respiratory syncytial virus (RSV) worldwide. However, molecular epidemiological data on RSV resurgence and genetic diversity in tropical regions during the post-pandemic period remain scarce. This study investigated the molecular epidemiology, seasonal trends, genetic diversity, and evolutionary dynamics of RSV circulating among Thai children with respiratory illness between 2023 and 2025. Of 5198 respiratory specimens, 697 (13.4%) tested positive for RSV, with a pronounced post-pandemic resurgence observed in 2023 (18.3%). RSV seasonality returned to its typical pre-pandemic pattern, peaking during the rainy season (July-November). A shift in age distribution was observed, with the highest burden occurring among 3-year-olds (27.0%). Alternating subgroup dominance was evident, with RSV-B predominating in 2023 (44.4%) and 2025 (43.9%), while RSV-A predominated in 2024 (46.3%). All RSV-A strains belonged to the ON1 genotype (A.D lineage), exhibiting sequential annual turnover of dominant sublineages from A.D.5.2 to A.D.1.4 and A.D.3. Conversely, RSV-B strains belonged to the BA9 genotype (B.D lineage) with sustained dominance of the B.D.E.1 sublineage (76.7%). Distinct lineage-specific amino acid substitutions suggested ongoing viral diversification and immune selection pressures. These findings underscore the importance of continued molecular surveillance to inform RSV prevention strategies.