Study of an Live-Attenuated Respiratory Syncytial Virus Vaccine in Infants and Toddlers

The primary objectives of the study were: * To assess the safety profile of each dose of the study product after each and any administration in all infants and toddlers regardless of baseline serostatus. * To characterize the Respiratory Syncytial Virus (RSV) A serum neutralizing antibody responses to the study product in each vaccine group after vaccination in RSV-naïve participants. The secondary objectives of the study were: * To quantify the amount of vaccine virus shed by each participant by baseline serostatus. * To determine the proportion of vaccinated infants and toddlers in each vaccine group infected with the vaccine virus at D56 (56 days after vaccination 1) for Cohorts 1, 2, 3 and 4, and at Day 84 (28 days after vaccination 2) for Cohorts 2 and 4 by baseline serostatus. * To characterize the RSV A serum neutralizing antibody responses to the study product in each vaccine group after vaccination in RSV-experienced participants. * To characterize serum RSV anti-F immunoglobulin G (IgG) antibody responses to the study product in each vaccine group after vaccination by baseline serostatus. * To characterize serum RSV antibody responses (RSV A-neutralizing and anti-RSV F IgG) to the study product in each vaccine group after the RSV surveillance season or at least 5 months after the last vaccine administration by baseline serostatus.