Birth Month Matters in Timing of Seasonal Prevention of Severe Respiratory Syncytial Virus

Aims Respiratory syncytial virus (RSV) is a leading cause of respiratory infections in infants and young children. To optimise preventive strategies, we investigate the association between birth month and severe RSV infection. Methods This study utilised national health discharge and microbiology registries to investigate the association between birth month and severe RSV leading to hospitalisation in Denmark 2015–2023 in a matched case–control design. Results A total of 634 148 hospital admissions were identified, including 30 304 RSV‐related admissions. The risk of RSV hospitalisation was significantly associated with sex, birth month, birth weight, gestational age, perinatal tobacco exposure, congenital disease. December was the birth month with the highest risk of RSV hospitalisation (OR: 2.7, CI 95%: 2.5–3.0), followed by November, January, October. Conclusion Birth month significantly influences the risk of severe RSV infections in children under 2 years of age in countries with a temperate climate and four annual seasons such as Denmark. Birth months of September through February are associated with significantly higher risks of being hospitalised with RSV infection. These insights should inform national RSV preventive strategies, including maternal vaccination and neonatal immunisation with monoclonal antibodies in countries with similar climates to optimise timing and efficacy.