Evaluating the Infectivity, Safety, and Immunogenicity of a Recombinant Live-Attenuated Respiratory Syncytial Virus Vaccine in RSV-Seronegative Infants and Children 6 to 24 Months of Age

Inclusion Criteria: * Greater than or equal to 6 months (greater than or equal to 180 days) of age at the time of screening and less than 25 months (less than 750 days) of age * Participant is in good health based on review of the medical record, history, and physical examination, without evidence of chronic disease * Parents/guardians are willing and able to provide written informed consent * Seronegative for RSV antibody, defined as a serum RSV-neutralizing antibody titer less than 1:40 at screening from a sample collected no more than 42 days prior to inoculation * Is growing at a normal velocity for age as demonstrated on a standard growth chart AND * If less than 1 year of age: has a current height and weight above the 5th percentile * If 1 year of age or older: has a current height and weight above the 3rd percentile for age * Participant has received routine immunizations appropriate for age (as per Center for Disease Control Advisory Committee on Immunization Practices \[ACIP\]) * Participant is expected to be available for the duration of the study Exclusion Criteria: * Known or suspected HIV infection or impairment of immunological functions * Receipt of immunosuppressive therapy, including any systemic, including either nasal or inhaled, corticosteroids within 28 days of enrollment. Note: Cutaneous (topical) steroid treatment is not an exclusion. * Bone marrow/solid organ transplant recipient * Major congenital malformations (such as congenital cleft palate) or cytogenetic abnormalities * Previous receipt of a licensed or investigational RSV vaccine or receipt of placebo in this study or previous receipt of or planned administration of any anti-RSV product (such as ribavirin or RSV IG) * Previous anaphylactic reaction * Previous vaccine-associated adverse reaction that was Grade 3 or above * Known hypersensitivity to any study product component * Heart disease. Note: Participants with cardiac abnormalities documented to be clinically insignificant and requiring no treatment may be enrolled. * Lung disease, including any history of reactive airway disease or medically documented wheezing * Member of a household that contains, or will contain, an infant who is less than 6 months of age at the enrollment date through Day 28 * Member of a household that contains another child who is, or is scheduled to be, enrolled in CIR 311, 312 or 313 AND there has been or will be an overlap in residency during that other child's participation in the study's Acute Phase (Days 0 to 28) * Member of a household that contains an immunocompromised individual, including but not limited to: * a person who is HIV infected * a person who has received chemotherapy within the 12 months prior to enrollment * a person receiving immunosuppressant agents * a person living with a solid organ or bone marrow transplant * Attends a daycare facility and shares a room with infants less than 6 months of age, and parent/guardian is unable or unwilling to suspend daycare for 28 days following inoculation * Any of the following events at the time of enrollment: * fever (rectal temperature of greater than or equal to 100.4°F (38°C)), or * upper respiratory signs or symptoms (rhinorrhea, cough, or pharyngitis) or * nasal congestion significant enough to interfere with successful inoculation, or * otitis media * Receipt of the following prior to enrollment: * any killed vaccine or live-attenuated rotavirus vaccine within the 14 days prior, or * any live vaccine, other than rotavirus vaccine, within the 28 days prior, or * another investigational vaccine or investigational drug within 28 days prior * Scheduled administration of the following after planned inoculation: * killed vaccine or live-attenuated rotavirus vaccine within the 14 days after, or * any live vaccine other than rotavirus in the 28 days after, or * another investigational vaccine or investigational drug in the 56 days after * Receipt of immunoglobulin, any antibody products, or any blood products within the past 6 months * Receipt of any of the following medications within 3 days of study enrollment: * systemic antibacterial, antiviral, antifungal, anti-parasitic, or antituberculous agents, whether for treatment or prophylaxis, or * intranasal medications, or * other prescription medication except as listed below * Receipt of salicylate (aspirin) or salicylate-containing products within the past 28 days * Born at less than 34 weeks gestation * Born at less than 37 weeks gestation and less than 1 year of age at the time of enrollment * Meets criteria for failure to thrive within the six months prior to enrollment: a decline in height or weight growth that has crossed two major growth percentiles (e.g., from above the 75th to below the 25th) in an interval of less than 6 months * Suspected or documented developmental disorder, delay, or other developmental problem * Previous receipt of supplemental oxygen therapy in a home setting