The purpose of this study is to determine whether vYF (investigational vaccine) is safe and can help the body to develop antibodies (immunogenicity) compared with Stamaril vaccine and YF-VAX vaccine (both licensed vaccines) and when they are co-administered with Measles Mumps Rubella (MMR) vaccines in infants aged 11-15 months.

Number of Participants:

A total of 2440 participants is planned to be enrolled in VYF04 study.

Study Arms and Duration:

Eligible participants will be randomized in 2 independent groups (9-24 months, 2-5 years) to receive 1 dose of either vYF or Stamaril or YF-VAX in a 2:1:1 ratio within each age group. An additional group with participants of 11-15 months of age will also receive at the same vaccination visit vYF and a single dose of MMR vaccine.

For the 2nd step (YF booster vaccine administration in a subset), at the Year (Y) 3 visit, a subset of 120 participants of the 9-24 months of age group who did receive a YF vaccine on Day(D) 01 will be invited to join a booster dose assessment (booster dose administered after the Y3 visit blood sample has been taken). Participants aged 11 to 15 months at the time of the concomitant administration of vYF and MMR will not be eligible for receiving a booster dose.

The duration of each participation will be approximately 3 years for all participants (including participants co-administered on D01 with vYF and MMR), and 6 more months post-booster dose administration for the participants enrolled in the booster subset.

The duration of each participant's participation will be up to approximately 3 years (not including booster phase in a subset)

The Phase III VYF04 is the first study to be carried out with the investigational vYF in pediatric populations.

Inclusion Criteria:

• Aged 9 months to 5 years on the day of inclusion*

  * "9 months to 5 years" means from the day of the 9th month after birth to the day before the 6th year birthday

• Aged 11 to 15 months* on the day of inclusion for participants enrolled in the MMR co-administration group

  * "11 to 15 months" means from the day of the 11th month after birth to the day before the 16th month birthday

• Participants who are healthy as determined by medical evaluation including medical history and physical examination

• For infants*, 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

  * Infants aged 9 months to 11 months up to the day before the 12th month birthday

• Participant and parent/LAR are able to attend all scheduled visits and to comply with all study procedures

• ICF has been signed and dated by the parent(s) or other LAR (and by an independent witness if required by local regulations)

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

• Known or suspected congenital or acquired immunodeficiency; or receipt of immunosuppressive therapy, such as anti-cancer chemotherapy irradiation 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 history of FV infection

• Known systemic hypersensitivity to any of the study intervention components, eggs or history of a life-threatening reaction to the study interventions used in the study or to a product containing any of the same substances

• Moderate or severe acute illness/infection (according to Investigator judgment) or febrile illness (temperature ≥ 38.0°C [≥ 100.4°F]) on the day of study intervention administration. A prospective participant should not be included in the study until the condition has resolved or the febrile event has subsided.

• Chronic illness* that, in the opinion of the Investigator, is at a stage where it might interfere with study conduct or completion , including malignancy, such as leukemia, or lymphoma

  *Chronic illness may include, but is not limited to, cardiac disorders, renal disorders, auto-immune disorders, diabetes, psychiatric disorders or chronic infection

• History of central nervous system disorder or disease, including seizures and febrile seizures

• Receipt of any vaccine in the 4 weeks preceding the study intervention administration or planned receipt of any vaccine in the 4 weeks following the study intervention administration. Vaccine to be administered as part of the National Immunization Schedule will be postponed after the D29 visit

• Previous vaccination against a FV disease at any time including YF with an investigational or marketed vaccine

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

• Administration of any anti-viral within 2 months preceding the study intervention administration and up to the 6 weeks following the study intervention administration

• For participants enrolled in the MMR co-administration group: previous vaccination against measles, measles/mumps/rubella

• For participants enrolled in the MMR co-administration group: history of measles, mumps, rubella confirmed either clinically, serologically, or microbiologically

• Known history or laboratory evidence of HIV infection

• Known history of hepatitis B or hepatitis C seropositivity

• Personal or family history of thymic pathology (thymoma, thymectomy, or myasthenia)

• Participation at the time of study enrollment (or in the 4 weeks preceding the study intervention administration) or planned participation during the first year of the 3-year follow-up in another clinical study investigating a vaccine, drug, medical device, or medical procedure. Enrollment in another study after the first 6 months of follow-up is permitted, assuming it does not exclude participation in this study.

• In an emergency setting, or hospitalized involuntary

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

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

• Sponsor, Investigators, Study staff who conduct the safety assessment and the participant' parent(s) / LAR(s) will not know which study intervention is administered.
• Only the study staff who prepare and administer the study interventions and are not involved with the safety evaluation will know which study intervention is administered.