RSV is the most common viral cause of serious acute lower respiratory illness (LRI) in infants and children under 5 years of age in the world. RSV illness can range from mild upper respiratory tract illness (URI) to severe LRI, including bronchiolitis and pneumonia. Severe RSV disease in infancy may also predispose children to develop reactive airway disease during childhood. The purpose of this study is to evaluate the safety and immunogenicity of an RSV vaccine (RSV LID ΔM2-2) in RSV-seronegative infants and children at least 6 months and through 24 months of age.