Human respiratory syncytial virus (RSV) is the most common viral cause of serious acute lower respiratory illness (LRI) in infants and children under 5 years of age worldwide. The purpose of this study is to evaluate the safety, infectivity, and immunogenicity of a single dose of a recombinant live-attenuated RSV vaccine, RSV LID ΔM2-2 1030s, in RSV-seronegative infants 6 to 24 months of age.