Prevalence and Genetic Diversity of Respiratory Syncytial Virus Reinfections in Young Thai Children, 2016–2023

Although a vaccine for respiratory syncytial virus (RSV) is now available for pregnant women and the elderly, RSV remains a significant cause of respiratory illness in children globally. Reinfections by the same or different RSV subgroups in children residing in the tropics are currently under-studied. Therefore, we examined the patterns of RSV infection and reinfection in Thai children aged ≤ 5 years with respiratory symptoms from 2016 to 2023. Screening of 7710 pediatric respiratory specimens identified 1245 RSV-positive samples (16.1%), mostly from the rainy months (July–November). Interestingly, 74 children experienced two infections, and 6 had three infections. Reinfection by different RSV subgroups occurred in 30 children: 21 were initially infected with RSV-B and later with RSV-A, while 9 had the reverse pattern. Reinfections only by either RSV-A or RSV-B were observed in 22 and 2 children, respectively, with one child infected with RSV-A three times. All RSV-A reinfections belonged to the ON1 genotype, while RSV-B reinfections were BA9. Notably, reinfections across different seasons were observed within homologous pairs. These findings suggest a transitory immunity to natural RSV infection and provide the knowledge that may help optimize pediatric vaccination schedule. Ongoing epidemiological data on RSV are essential in monitoring genotype circulation and vaccine effectiveness.