An AE was development of any untoward medical occurrence in a participant or clinical study participant administered medicinal product and which did not necessarily have causal relationship with this treatment. TEAEs were AEs whose onset occurred after receiving nirsevimab through 360 days post second dose. An SAE was any AE that resulted in death, was immediately life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent or significant disability/incapacity, was a congenital anomaly or birth defect or was an important medical event that might jeopardize the participant or may require medical treatment to prevent 1 of the outcomes listed above. AESIs were based on assessment by investigators following the administration of nirsevimab. An NOCD was a newly diagnosed medical condition of chronic, ongoing nature post administration of study drug.