The primary objectives of the study were:

• To assess the safety profile of each dose of the study product after each and any administration in all infants and toddlers regardless of baseline serostatus.
• To characterize the Respiratory Syncytial Virus (RSV) A serum neutralizing antibody responses to the study product in each vaccine group after vaccination in RSV-naïve participants.

The secondary objectives of the study were:

• To quantify the amount of vaccine virus shed by each participant by baseline serostatus.
• To determine the proportion of vaccinated infants and toddlers in each vaccine group infected with the vaccine virus at D56 (56 days after vaccination 1) for Cohorts 1, 2, 3 and 4, and at Day 84 (28 days after vaccination 2) for Cohorts 2 and 4 by baseline serostatus.
• To characterize the RSV A serum neutralizing antibody responses to the study product in each vaccine group after vaccination in RSV-experienced participants.
• To characterize serum RSV anti-F immunoglobulin G (IgG) antibody responses to the study product in each vaccine group after vaccination by baseline serostatus.
• To characterize serum RSV antibody responses (RSV A-neutralizing and anti-RSV F IgG) to the study product in each vaccine group after the RSV surveillance season or at least 5 months after the last vaccine administration by baseline serostatus.

Study duration per participant was maximum 12 months

Inclusion Criteria:

Inclusion criteria :

• Aged 6 through 18 months at Day 0.
• Informed consent form has been signed and dated by the parent(s) or other legally acceptable representative (and by independent witness if required by local regulations).
• Participant and parent / guardian / legally acceptable representative are able to attend all scheduled visits and to comply with all trial procedures

Exclusion Criteria:

• Born at less than 34 weeks gestation

• Born at less than 37 weeks gestation and less than 1 year of age at the time

• 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).

• Probable or confirmed case of Coronavirus Disease 2019 (COVID-19).

• Known systemic hypersensitivity to any of the vaccine components, or history of a life-threatening reaction to the vaccine used in the trial or to a vaccine containing any of the same substances

• Any chronic illness

  • Chronic illness may include, but is not limited to, cardiac disorders, lung disease (including any history of reactive airway disease, receipt of bronchodilator therapy, or medically diagnosed wheezing), renal disorders, auto-immune disorders, diabetes, psychomotor diseases, and known congenital or genetic diseases

• Any history of medically diagnosed wheezing

• Any acute febrile, respiratory or gastrointestinal illness in the past 24 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

• Any previous anaphylactic reaction

• Current suspected or documented developmental disorder, delay, or other developmental problem

• Receipt of any of the following vaccines prior to enrollment:

  • any influenza vaccine within 7 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

• Previous receipt of a licensed or investigational RSV vaccine or previous receipt or planned administration of any anti-RSV product (such as ribavirin or RSV immune immune globulins [IG] or RSV monoclonal antibody)

• Receipt of immune globulins, blood or blood-derived products in the past 6 months prior to enrolment

• Receipt of any of the following medications within 3 days prior to study enrollment (Day 0):

  • systemic antibacterial, antiviral, antifungal, anti-parasitic, or antituberculous agents, whether for treatment or prophylaxis, or
  • intranasal medications, or
  • other prescription medication except as permitted concomitant medications (prescription or non-prescription) including 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 (Day 0)

• Any previous vaccine-associated adverse reaction that was Grade 3 or above. Note: if grading is not possible, determine if the reaction was considered severe or life threatening; if so, it is exclusionary.

• Scheduled administration of the following after planned inoculation:

  • any influenza vaccine within 7 days after, or
  • inactivated 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.

• Any previous receipt of supplemental oxygen therapy in a home or hospital setting, except the temporary receipt of supplemental oxygen for transient tachypnea in newborn

• 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

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

• Member of a household that contains, or will contain, an infant who is less than 6 months of age at the enrollment date (or in the 6 weeks preceding the first trial vaccination) through Day 28

• Member of a household that contains another child/other children who is/are, or is/are scheduled to be, enrolled in this study in the same year AND the date of enrollment will not be concurrent with the other participant(s) living in the household (i.e., all eligible children from the same household must be enrolled on the same date)

• Attends a daycare facility and shares a daycare room with infants less than 6 months of age, and parent/guardian is unable or unwilling to suspend daycare for 28 days following inoculation

• 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 was not intended to contain all considerations relevant to a participant's 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

The study was performed in an observer-blind fashion. Investigators and study staff who conducted the safety assessment and the participants did not know which vaccine is administered. Only the study staff who prepare and administer the vaccine and were not involved with the safety evaluation knew which vaccine was administered.