Evaluating the Infectivity, Safety and Immunogenicity of Respiratory Syncytial Virus Vaccines, RSV 6120/∆NS1 and RSV 6120/F1/G2/∆NS1, in RSV-Seropositive Children and RSV-Seronegative Infants and Children

INCLUSION CRITERIA FOR RSV-SEROPOSITIVE CHILDREN * Greater than or equal to 12 months of age and less than 60 months of age at the time of inoculation * Screening serum specimen for RSV-neutralizing antibody is obtained within the calendar year of inoculation * Seropositive for RSV antibody, defined as serum RSV-neutralizing antibody titer greater than or equal to 1:40 * Pre-inoculation serum sample for RSV-neutralizing antibody specimen is obtained no more than 56 days prior to inoculation * In good health based on review of the medical record, history, and physical examination (PE) at the time of inoculation * Received routine immunizations appropriate for age based on the Advisory Committee on Immunization Practices (ACIP) Recommended Immunization Schedule for Children and Adolescents Aged 18 Years or Younger * Growing normally for age as demonstrated on a standard growth chart and has a current height and weight above the 3rd percentile for age * Expected to be available for the duration of the study * Parent/guardian is willing and able to provide written informed consent EXCLUSION CRITERIA FOR RSV-SEROPOSITIVE CHILDREN * Born at less than 34 weeks gestation * Maternal history of positive human immunodeficiency virus (HIV) test * Evidence of chronic disease * Known or suspected impairment of immune function * Bone marrow/solid organ transplant recipient * Major congenital malformations, including congenital cleft palate or cytogenetic abnormalities * Suspected or documented developmental disorder, delay, or other developmental problem * Cardiac abnormality requiring treatment * Lung disease or reactive airway disease * More than one episode of wheezing in the first year of life * Wheezing episode or received bronchodilator therapy within the past 12 months * Wheezing episode or received bronchodilator therapy after the age of 12 months * Previous receipt of supplemental oxygen therapy in a home setting * Previous receipt of an investigational RSV vaccine * Previous receipt or planned administration of anti-RSV antibody product including ribavirin, RSV Ig or RSV mAb * Previous receipt of immunoglobulin or any antibody products within the past 6 months * Previous receipt of any other blood products within the past 6 months * Previous anaphylactic reaction * Previous vaccine-associated adverse reaction that was Grade 3 or above * Known hypersensitivity to any vaccine component * Member of a household that contains an infant who is less than 12 months of age at the date of inoculation through the 10th day after inoculation * Member of a household that, at the date of inoculation through the 10th day after inoculation, 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 * Will attend a daycare facility that does not separate children by age and contains an infant who is less than 12 months of age at the date of inoculation through the 10th day after inoculation * Receipt of any of the following prior to enrollment: * any inactivated 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, or * salicylate (aspirin) or salicylate-containing products within the past 28 days * Scheduled administration of any of the following after planned inoculation: * inactivated vaccine or live-attenuated rotavirus vaccine within the 14 days after, or * any live vaccine other than rotavirus within the 28 days after, or * another investigational vaccine or investigational drug within the 28 days after * 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 medications except the permitted concomitant medications listed in the protocol * Any of the following events at the time of enrollment: * fever (temporal or rectal temperature of greater than or equal to 100.4°F), or * upper respiratory signs or symptoms (rhinorrhea, cough, or pharyngitis) or * nasal congestion significant enough to interfere with successful inoculation, or * otitis media INCLUSION CRITERIA FOR RSV-SERONEGATIVE INFANTS \& CHILDREN * Greater than or equal to 6 months of age and less than 25 months of age at the time of inoculation * Screening and pre-inoculation serum specimens for RSV-neutralizing antibody are obtained no more than 42 days prior to inoculation * Seronegative for RSV antibody, defined as serum RSV-neutralizing antibody titer less than 1:40 * In good health based on review of the medical record, history, and PE at the time of inoculation * Received routine immunizations appropriate for age based on the ACIP Recommended Immunization Schedule for Children and Adolescents Aged 18 Years or Younger * Growing normally 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 for age * If greater than or equal to 1 year of age: has a current height and weight above the 3rd percentile for age * Expected to be available for the duration of the study * Parent/guardian is willing and able to provide written informed consent EXCLUSION CRITERIA FOR RSV-SERONEGATIVE INFANTS \& CHILDREN * Born at less than 34 weeks gestation * Born at less than 37 weeks gestation, and at the date of inoculation less than 1 year of age * Maternal history of a positive HIV test * Evidence of chronic disease * Known or suspected infection or impairment of immunological functions * Bone marrow/solid organ transplant recipient * Major congenital malformations, including congenital cleft palate or cytogenetic abnormalities * Suspected or documented developmental disorder, delay, or other developmental problem * Cardiac abnormality requiring treatment * Lung disease or reactive airway disease * More than one episode of wheezing in the first year of life * Wheezing episode or received bronchodilator therapy within the past 12 months * Wheezing episode or received bronchodilator therapy after the age of 12 months * Previous receipt of supplemental oxygen therapy in a home setting * Previous receipt of an investigational RSV vaccine * Previous receipt or planned administration of anti-RSV antibody product including ribavirin, RSV Ig, or RSV mAb * Previous receipt of immunoglobulin or any antibody products within the past 6 months * Previous receipt of any blood products within the past 6 months * Previous anaphylactic reaction * Previous vaccine-associated adverse reaction that was Grade 3 or above * Known hypersensitivity to any study product component * Member of a household that contains an infant who is less than 6 months of age at the date of inoculation through the 28th day after inoculation * Member of a household that, at the date of inoculation through the 28th day after inoculation, 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 that does not separate children by age and contains an infant less than 6 months of age at the date of inoculation through the 28th day after inoculation * Receipt of any of the following prior to enrollment: * any inactivated influenza vaccine within 3 days prior, or * any inactivated 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, or * salicylate (aspirin) or salicylate-containing products within the past 28 days * Scheduled administration of any of the following after planned inoculation * inactivated vaccine or live-attenuated rotavirus vaccine within the 14 days after, or * any live vaccine other than rotavirus within the 28 days after, or * another investigational vaccine or investigational drug within the 56 days after * 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 medications except the permitted concomitant medications 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. * Any of the following events at the time of enrollment: * fever (temporal or rectal temperature of greater than or equal to 100.4°F), or * upper respiratory signs or symptoms (rhinorrhea, cough, or pharyngitis) or * nasal congestion significant enough to interfere with successful inoculation, or * otitis media * contact with a person diagnosed with COVID-19 disease or active SARS-CoV-2 infection within the past 10 days.