Inclusion Criteria:  Age at the time of informed consent:  Part A: 2 to 5 years, inclusive Part B: 6 months to < 2 years  RSV Status at Screening:  Part A: RSV-seropositive Part B: RSV-seronegative Good general health status Product of normal full-term pregnancy (36 to 42 weeks gestation)  Exclusion Criteria:  Household contact with any of the following groups of individuals for the period up to 14 days after each dose:  Pregnant women Infants < 6 months of age With hospitalization for asthma or other chronic respiratory disease in the past 5 years  Immunocompromised individuals, which includes, but is not limited to, those with the following conditions:  AIDS Receipt of chemotherapy within the past 6 months Current receipt of immunosuppressive agents Solid organ or bone marrow transplant Enrolled in the same classroom at full-time day care with infants < 6 months of age for 14 days after each dose Household contact of another child enrolled into the study Inadequate venous access for repeated phlebotomy Height and weight ≤ 5th percentile for age and sex (according to CDC growth charts for children in Part A, according to World Health Organization Child Growth Standards for children in Part B)