Importance Respiratory syncytial virus (RSV) is the leading cause of infant hospitalizations, and preterm infants and those with older siblings are at increased risk. Despite recommendations for nirsevimab prophylaxis, data on outcomes in high-risk infants are lacking. Objective To assess the association of nirsevimab prophylaxis with RSV-related lower respiratory tract infection (LRTI) hospitalization risk and in-hospital severity among infants, overall and by high-risk groups. Design, Setting, and Participants T…