VYF03 is a phase II, randomized, parallel-group prevention study with 2 arms, active-controlled (Stamaril), observer-blind, multi-center study to assess the non-inferiority of the immune response, in terms of seroconversion rates of the investigational vaccine candidate vYF to the licensed Stamaril, in adults aged 18 years up to 60 years in Europe (EU).

The safety and immunogenicity profile of vYF in a cohort of Asian population of Chinese origin outside of China will also be described. The study will also assess the immunogenicity profiles and the safety profiles of vYF and Stamaril.

Participants will be randomized in a 2:1 ratio to receive a single subcutaneous injection of either the vYF vaccine (380 participants in EU and 80 participants of Chinese origin in Asia) or Stamaril (190 participants in EU and 40 participants of Chinese origin in Asia), on Day 01.

The duration of each participant's participation will be approximately 5 years.

The duration of each participant's participation will be approximately 5 years.

Inclusion Criteria:

• Aged 18 years up to 60 years* on the day of inclusion
• A female participant is eligible to participate if she is not pregnant or breastfeeding and one of the following conditions applies:

Is of non-childbearing potential. To be considered of non-childbearing potential, a female must be postmenopausal for at least 1 year, or surgically sterile OR Is of childbearing potential and agrees to use an effective contraceptive method (1) or abstinence (1) from at least 4 weeks prior to study intervention administration until at least 4 weeks (2) after study intervention administration.

*18 to 60 years means from the day of the 18th birthday up to the day before the 60th birthday

1. Not applicable for Finland

2. Except for French participants which have to apply 12 weeks contraception after study intervention administration A female participant of childbearing potential must have a negative highly sensitive pregnancy test (urine or serum as required by local regulation) before any dose of study intervention on Day 1 and the test will be repeated on D29 to confirm the participant is still not pregnant within 28 days of vaccine administration.

   - Informed consent form has been signed and dated (3)

3. For participants aged less than 21 years in Singapore, an informed consent form has been signed and dated by both the participant and the parent(s) or another legally acceptable representative

   • Able to attend all scheduled visits and to comply with all study procedures
   • For participants enrolled in Asian countries as part of the additional cohort only: know Chinese origin, defined as having at least one biological parent of Chinese origin, and will be self-reported by the participant

   Exclusion Criteria:

   Participation at the time of study enrollment (or in the 4 weeks preceding the study vaccination) or planned participation during the first 2 years of the 5-year follow-up in another clinical study investigating a vaccine, drug, medical device, or medical procedure. Enrollment in another study after the first 2 years is permitted, assuming that it does not exclude participation in this study.

   • Receipt of any vaccine in the 4 weeks preceding the study vaccination or planned receipt of any vaccine in the 4 weeks following the study vaccination (prior to visit 4), except for influenza vaccination, which may be received at least 2 weeks before study vaccines (4). This exception includes all influenza vaccines including monovalent pandemic influenza vaccines.
   • Previous vaccination against a FV disease at any time including YF with either the study vaccine or another vaccine.
   • Receipt of immune globulins, blood, or blood-derived products in the past 6 months.
   • 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 history of any FV infection.
   • Known systemic hypersensitivity to any of the vaccine components, eggs, or history of a life-threatening reaction to the vaccines used in the study or to a vaccine containing any of the same substances.
   • Known history or laboratory evidence of HIV infection (5).
   • Known history or laboratory evidence of hepatitis B or hepatitis C infection (6).
   • Personal or family history of thymic pathology (thymoma, thymectomy, or myasthenia).
   • Deprived of freedom by an administrative or court order, or in an emergency setting, or hospitalized involuntarily.
   • Alcohol, prescription drug, or substance abuse that, in the opinion of the Investigator, might interfere with the study conduct or completion.
   • Chronic illness (7) that, in the opinion of the Investigator, is at a stage where it might interfere with trial conduct or completion, including malignancy, such as leukemia, or lymphoma.
   • Moderate or severe acute illness/infection (according to Investigator judgment) on the day of vaccination or febrile illness (temperature ≥ 100.4°F or 38°C). A prospective participant should not be included in the study until the condition has resolved or the febrile event has subsided.
   • Administration of any anti-viral within 2 months preceding the vaccination and planned administration up to the 6 weeks following the vaccination.
   • Identified as an Investigator or employee of the Investigator or study center with direct involvement in the proposed study, or identified as an immediate family member (ie, parent, spouse, natural or adopted child) of the Investigator or employee with direct involvement in the proposed study.
   • Planned travel in a YF endemic country within 6 months of investigational or control vaccine administration.

4. Except for Thai participants

5. HIV Serology testing will be performed on all German participants if no evidence of seronegativity in the 90 days preceding vaccination

6. Hepatitis B and Hepatitis C Serology testing will be performed on all German participants if no evidence of seronegativity in the 90 days preceding vaccination

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

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

Investigators, Sponsor, and Study staff who conduct the safety assessment and the participant will not know which vaccine is administered. Only the study staff who prepare and administer the vaccine and are not involved with the safety evaluation ("vaccinator") will know which vaccine is administered.