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

The purpose of this study is to measure the safety, PK, occurrence of ADA to nirsevimab, and anti-RSV neutralizing Ab in Japanese children with certain health conditions or pre-term infants aged ≤12 months. Study details include * The study duration is approximately 21 months with a 2-month enrollment period. * Study intervention is 2 doses administered 5- 6 months apart. * The study has 5 or 6 site visits and several telephone contacts with a 2 or 4 week interval.