This is a Phase 3b, prospective, multicenter, open-label, non-controlled study to assess the safety and effectiveness of immunoprophylaxis with the intramuscular (IM) administration of the liquid formulation of palivizumab for the prevention of RSV hospitalizations in infants at high risk (infants born at less than or equal to 35 weeks gestational age and less than or equal to 6 months of age at enrollment; or infants less than or equal to 24 months of age with a diagnosis of chronic lung disease [CLD] of prematurity requiring on-going medical treatment within the previous 6 months or infants less than or equal to 24 months of age with hemodynamically significant congenital heart disease [CHD]).

Participants will receive palivizumab solution for injection at 15 mg/kg by IM injection every 30 days for a minimum of 3 and a maximum of 5 injections given during anticipated periods of RSV risk in the community; the number of doses will depend on the time of enrollment during the RSV season.

Inclusion Criteria:

Infants at high risk of severe RSV infection defined as fulfilling at least one of the following:

• Infants born ≤ 35 weeks gestational age AND are ≤ 6 months of age at enrollment
• Infants ≤ 24 months of age at enrollment AND with a diagnosis of bronchopulmonary dysplasia (BPD) (defined as oxygen requirement at a corrected gestational age of 36 weeks) requiring intervention/management (i.e., oxygen, diuretics, bronchodilators, corticosteroids, etc.) anytime within 6 months prior to enrollment
• Infants ≤ 24 months of age at enrollment with hemodynamically significant CHD, either cyanotic or acyanotic, unoperated or partially corrected. Children with acyanotic cardiac lesions must have pulmonary hypertension (≥ 40 mmHg measured pressure in the pulmonary artery [ultrasound acceptable]) or the need for daily medication to manage hemodynamically significant CHD

Exclusion Criteria (main exclusion criteria):

• Hospitalization at the time of enrollment (unless discharge is anticipated within 14 days)
• Mechanical ventilation (including continuous positive airway pressure, CPAP) at the time of enrollment
• Life expectancy less than 6 months
• Unstable cardiac or respiratory status, including cardiac defects so severe that survival is not expected or for which cardiac transplantation is planned or anticipated
• Active respiratory illness, or other acute infection