Study on a Live-attenuated Respiratory Syncytial Virus Vaccine for Assessment of Safety, Transmissibility, and Genetic Stability of the Vaccine Virus Among Close Contacts in Infants and Toddlers 6 to < 24 Months of Age in Puerto Rico (USA)

Inclusion Criteria

• Aged 6 months to < 24 months on the day of inclusion (from the day of the 6 months after birth to the day before the 2nd birthday)
• Participants who are healthy as determined by medical evaluation including medical history.
• Born at full term of pregnancy (≥ 37 weeks) or born after a gestation period of 27 through 36 weeks and medically stable as assessed by the investigator, based on the following definition: "Medically stable" refers to the condition of premature infants who do not require significant medical support or ongoing management for debilitating disease and who have demonstrated a clinical course of sustained recovery by the time they receive the first dose of study intervention
• Attends a daycare facility at least 3 days per week and 4 hours per day at which the participant would be in a contact group/playroom of at least one other child 6 to < 24 months of age who will participate in this study or is a member of a household, which includes at least one other child 6 to < 24 months of age who will participate in this study

Exclusion Criteria

• Known or suspected congenital or acquired immunodeficiency; or receipt of immunosuppressive therapy, such as anti-cancer chemotherapy or radiation therapy, within the preceding 6 months; or long-term systemic corticosteroid therapy (prednisone or equivalent for more than 2 consecutive weeks within the past 3 months)
• Known systemic hypersensitivity to any of the study intervention components, or history of a life-threatening reaction to the study intervention used in the study or to a product containing any of the same substances
• Chronic illness that, in the opinion of the investigator, is at a stage where it might interfere with study conduct or completion
• Any acute febrile illness in the past 48 hours that according to investigator judgment is significant enough to interfere with successful inoculation on the day of vaccination. A prospective participant should not be included in the study until the condition has resolved or the febrile event has subsided.
• Probable or confirmed ongoing case of COVID-19 at the time of enrollment
• 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 study enrollment
• a person who has received (within the past 6 months) or is receiving (at the time of enrollment) immunosuppressant agents
• a person living with a solid organ or bone marrow transplant
• Member of a household that includes, or will include, an infant who is less than 6 months of age at the time of enrollment
• Attends a daycare facility and shares a daycare room with infants less than 6 months of age, and parent/legally acceptable representative is unable or unwilling to suspend attendance at the daycare facility for 28 days following study intervention administration
• Any need of supplemental oxygen therapy in a home or hospital setting at the time of enrollment.
• Participant's mother previous receipt or planned administration of an investigational RSV vaccine or any monoclonal antibody (such as Infliximab) during pregnancy and/or breastfeeding.
• Receipt or planned receipt of any of the following vaccines prior to or after the first study intervention administration:
• any influenza vaccine within 7 days prior to and after, or any COVID-19 or inactivated vaccine or live-attenuated rotavirus vaccine within 14 days prior to and after, or
• any live vaccine, other than rotavirus vaccine, within 28 days prior to and after
• Previous receipt of an investigational RSV vaccine or receiving any anti-RSV product (such as ribavirin or RSV Immunoglobulin (IG) or RSV monoclonal antibody) at the time of enrollment.
• Receipt of immune globulins, blood or blood-derived products in the past 3 months
• Receipt of intranasal and intra-ocular medications within 3 days prior to study enrollment
• Receipt at