Inclusion Criteria:

• 24 months of age or younger at randomization (child must be randomized on or before their 24-month birthday)
• Documented, hemodynamically significant CHD Note: Children with uncomplicated small atrial or ventricular septal defects or patent ductus arteriosus are not eligible
• Unoperated or partially corrected CHD
• Written informed consent obtained from the patient’s parent(s) or legal guardian

Exclusion Criteria:

• Unstable cardiac or respiratory status, including cardiac defects so severe that survival is not expected or for which cardiac transplantation is planned or anticipated
• Hospitalization, unless discharge is anticipated within 21 days
• Anticipated cardiac surgery within two weeks of randomization
• Requirement for mechanical ventilation, extracorporeal membrane oxygenation, continuous positive airway pressure or other mechanical respiratory or cardiac support
• Associated non-cardiac anomalies or end organ dysfunction resulting in anticipated survival of less than six months or unstable abnormalities of end organ function
• Acute respiratory or other acute infection or illness (children with respiratory symptoms must have a negative RSV test prior to randomization)
• Chronic seizure or evolving or unstable neurologic disorder
• Known immunodeficiency
• Mother with HIV infection (unless the child has been proven to be not infected)
• Known allergy to Ig products
• Receipt of palivizumab, RSV-IGIV, or other RSV-specific monoclonal antibody, or any other polyclonal antibody (for example, Hepatitis B IG, IVIG, VZIG) within 3 months prior to randomization
• Use of investigational agents within the past three months (other than investigational agents commonly used during cardiac surgery or the immediate post-operative period, e.g., nitric oxide)
• Current participation in other investigational protocols of drugs or biological agents