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

Inclusion Criteria: * Greater than or equal to 6 months (defined as greater than or equal to 180 days) of age at the time of screening and less than 25 months (defined as less than 750 days) of age. * Good health based on review of the medical record, history, and physical examination, without evidence of chronic disease. * Parents/guardians willing and able to provide written informed consent as described in the protocol. * 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. * Growing at a normal velocity for age (as demonstrated on a standard growth chart) AND * If less than 1 year of age: current height and weight above the 5th percentile * If 1 year of age or older: current height and weight above the 3rd percentile for age. * Received routine immunizations appropriate for age (as per national Center for Disease Control Advisory Committee on Immunization Practices \[ACIP\]). * Expected to be available for the duration of the study. * If born to an HIV-infected woman, participant must not have been breastfed and must have had documentation of 2 negative HIV nucleic acid (RNA or DNA) test results from samples collected on different dates with both collected when greater than or equal to 1 month of age and at least one collected when greater than or equal to 4 months of age, and no positive HIV nucleic acid (RNA or DNA) test; or 2 negative HIV antibody tests, both from samples collected at greater than or equal to 6 months of age. 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 was 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 placebo in the IMPAACT 2011 study) or previous receipt of or planned administration of any anti-RSV product (such as ribavirin or RSV IG or RSV mAb). * 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 contained, or would 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 IMPAACT 2011, 2012 or 2013 AND there had been or would 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 was greater than or equal to 6 years of age with HIV-related immunodeficiency, defined as having a most recent CD4 T lymphocyte cell count less than 300 cells/mm\^3. CD4 T lymphocyte count must have been measured within 6 months prior to enrollment, or * a person age 1 year up to less than 6 years with HIV-related immunodeficiency, defined as having a most recent CD4 T lymphocyte cell percentage less than 25 or CD4 T lymphocyte count less than 750 cells/mm\^3 (if both values are available, use the lower of the two). CD4 T lymphocyte parameter must have been measured within the 6 months prior to enrollment; or * a person age less than 1 year with HIV-related immunodeficiency, defined as having a most recent CD4 T lymphocyte cell percentage less than 30 or CD4 T lymphocyte count less than 1000 cells/mm\^3 (if both values are available, use the lower of the two). CD4 T lymphocyte parameter must have been measured within the 6 months prior to enrollment; or * a person who had received chemotherapy within the 12 months prior to enrollment; or * a person receiving immunosuppressant agents; or * a person living with a solid organ or bone marrow transplant. Verbal report of CD4 T cell lymphocyte is sufficient documentation if the parent/guardian is confident of history. * 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. Permitted concomitant medications (prescription or non-prescription) include nutritional supplements, medications for gastroesophageal reflux, eye drops, and topical medications, including (but not limited to) cutaneous (topical) steroids, topical antibiotics, and topical antifungal agents. * Receipt of salicylate (aspirin) or salicylate-containing products within the 28 days prior to enrollment. * 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.