A Study to Assess Safety, Pharmacokinetics Anti-Drug Antibody and Anti-RSV Antibody After 2 Doses of Nirsevimab

Inclusion Criteria: 1. Written informed consent and any locally required authorization obtained from the participant's parent(s)/legally authorized representative(s) before performing any protocol-related procedures, including screening evaluations 2. Japanese infants of ≤12 months of age eligible to receive palivizumab in accordance with national or local guidelines and those who must meet at least one of the following conditions at the time of informed consent. 1. Immunodeficiency 2. Chronic Lung Disease 3. Congenital Heart Disease 4. Down syndrome 5. Born pre-term ≤28 wks Gestation age and aged ≤12 months, or born pre-term \>28 wks and ≤35 wks Gestation age and aged ≤6 months 3. The participant's parent(s)/legally authorized representative(s) can understand and comply with the requirements of the protocol including follow-up visits as judged by the investigator. 4. The participant is available to complete the follow-up period for approximately 19 months, which will be approximately 1 year after receipt of 2nd dose of nirsevimab Exclusion Criteria: 1. Requirement for mechanical ventilation, extracorporeal membrane oxygenation, continuous positive airway pressure (CPAP), or other mechanical respiratory or cardiac support at the time of enrollment 2. A current, active RSV infection at the time of screening and investigational product administration 3. Any fever (≥100.4°F \[≥38.0°C\], regardless of route) or acute illness at the time of prior to investigational product administration 4. Any serious concurrent medical condition (except those resulting in an immune deficiency condition), including: 1. Known renal impairment 2. Known hepatic dysfunction including known or suspected active or chronic hepatitis infection 3. Any seizure disorder or evolving or unstable neurological condition 5. Anticipated cardiac surgery within 5-6 months after enrollment 6. Prior history of a suspected or actual acute life-threatening event 7. Receipt or intended use of palivizumab in the current enrollment season 8. Any known allergy or history of allergic reaction to any component of nirsevimab 9. Any known allergy or history of allergic reaction to immunoglobulin products, blood products, or other foreign proteins 10. Concurrent enrollment in another interventional study, or prior receipt of any investigational agent 11. Anticipated survival of less than 1 year at the time of informed consent 12. Any condition that, in the opinion of the investigator, would interfere with the evaluation of the investigational product or interpretation of study results 13. Children of employees of the Sponsor, clinical study site, or any other individuals involved with the conduct of the study, or immediate family members of such individuals