Distribution of RSV subtypes by participant age.

Objective There is conflicting evidence whether subtypes of Respiratory syncytial virus have different seasonality or are differentially associated with clinical severity. We aimed to explore the associations between disease severity and RSV subtypes RSV-A and RSV-B and to describe the circulation of RSV subtypes pattern by season and age. Methods Active prospective hospital surveillance for RSV-A and RSV-B in children <59 months of age was conducted during 2015–2018. All febrile children 12–59 months of age were enrolled, whereas children <12 months were eligible if presenting with fever or respiratory symptoms. Risk factors and upper and lower respiratory tract infection was identified by linkage to national registry data and analyzed using penalized maximum likelihood logistic regression. Results Both RSV-A and B were found to co-circulate throughout all three study seasons, and no clear seasonal pattern was identified. Likewise, we found no association between sex or measures of severity with RSV-A or RSV-B. There was significantly more RSV-A than RSV-B among children with comorbidities. Conclusions No association was found between disease severity or sex and RSV subtypes RSV-A and RSV-B in hospitalized young children in Norway.