Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in children worldwide, with nearly all children infected by the age of three. Although the majority of cases are managed in outpatient care, RSV is responsible for over 3.5 million global hospitalizations annually among children less than five years old. However, knowledge regarding RSV infections in youngest infants and sex-related differences remains limited. The aim of this study was to determine the burden and clinical features of RSV in children.

In a prospective outpatient cohort study, children under 10 years of age were followed over two consecutive respiratory seasons. During each infection, they were clinically examined, and RSV viral load was measured from nasopharyngeal samples. Parents filled daily symptom diaries during the study. We found that higher RSV viral load was associated with a longer duration of symptoms from the age of two years onwards, although no clear link was observed with complications or antibiotic use.

Retrospective inpatient studies investigated laboratory-confirmed RSV hospitalizations at Turku University Hospital in 2006-2020. Cases were analyzed on a population basis, and data were obtained from patient medical records. We found that RSV hospitalization rates peaked at one month of age and declined with increasing age. Among infants under three months, hospitalization rates were up to twice as high as previously reported. Additionally, boys were hospitalized up to 50% more often than girls during early childhood, except in the first months of life. Boys also more frequently presented with respiratory distress, while no differences were found in other clinical features, complications, antibiotic use, or length of hospital stay.

Our findings indicate that the burden of RSV hospitalization in the first months of life is considerably greater than previously assumed, emphasizing the importance of preventive strategies during early infancy. The observed sex differences in disease burden and clinical presentation may offer insights into the pathogenesis of RSV. Furthermore, the association between viral load and symptom duration highlights the clinical relevance of antiviral therapy development.