The primary purpose of the study is to assess the shedding, transmission, and genetic stability of the live-attenuated RSVt vaccine after each intranasal vaccination (56 days apart) in infants and toddlers 6 to < 24 months of age.

The duration of each participant's participation is up to 8 months, including the 6 months safety follow-up phone call after the second study intervention administration for the pediatric participants The treatment administration for the pediatric participants will be on D01 and D57 (1 intranasal administration each).

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 the time of enrollment or previous receipt of salicylate (aspirin) or salicylate-containing products

• Participation at the time of study enrollment (or in the 6 weeks preceding the first study intervention administration) or planned participation during the present study period in another clinical study investigating a vaccine, drug, medical device, or medical procedure

• Deprived of freedom in an emergency setting, or hospitalized involuntarily

• Identified as a natural or adopted child of the Investigator or employee with direct involvement in the proposed study

The above information is not intended to contain all considerations relevant to a potential participation in a clinical trial.

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org

Observer-blind:

• Blinding for vaccine group assignment: participants, parents or legally acceptable representative (LAR), outcome assessors, investigators, laboratory personnel, Sponsor study staff
• No blinding for study staff who prepare and administer the study interventions