The purpose of this study is to assess the safety of vYF (investigational vaccine) compared to the safety of YF-VAX (YF licensed vaccine) in adults aged 18 years up to 60 years.

Study details include:

• The study duration will be up to approximately 6 months.
• One single dose of the study intervention (vYF or YF-VAX) will be administered subcutaneously at the 1st visit.
• The visit(V) frequency will be Day(D) 01 (V01) and D29 (V02). Two telephone calls will be planned at D15 and Month(M)6.

Number of Participants:

A total of 640 participants are expected to be included in the study in a 3:1 ratio (vYF: YF-VAX):

Group 1: vYF; N=480 Group 2: YF-VAX; N=160

Study Arms and Duration:

Randomized, active control (YF-VAX). 6 months duration per participants. Eligible participants will be randomized in a 3:1 ratio to receive, subcutaneously, a single dose of vYF or YF-VAX vaccines on D01.

Inclusion Criteria:

• Aged from 18 years to 60 years on the day of inclusion

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

• 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 post-menopausal for at least 1 year, or surgically sterile.

OR

-Is of childbearing potential and agrees to use an effective contraceptive method or abstinence from at least 4 weeks prior to study intervention administration until at least 12 weeks 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 D01 and will be repeated on D29 to confirm the participant is still not pregnant within the 28 days of vaccine administration
• Able to attend all scheduled visits and to comply with all study procedures Informed consent
• Informed consent form has been signed and dated

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 history of laboratory evidence of HIV infection
• Known history of hepatitis B or hepatitis C seropositivity
• Known history of flavivirus (FV) infection
• Known systemic hypersensitivity to any of the study intervention components, or history of a life-threatening reaction to the vaccine used in the study or to a vaccine containing any of the same substances
• Personal or family history of thymic pathology (thymoma, thymectomy, or myasthenia)
• Chronic illness 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 ≥ 38°C [or ≥ 100.4°F]). A prospective participant should not be included in the study until the condition has resolved or the febrile event has subsided.
• Alcohol, prescription drug, or substance abuse that, in the opinion of the Investigator, might interfere with the study conduct or completion
• 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 2 on D29), except for influenza vaccination, which may be received at least 2 weeks before study vaccine. This exception includes pandemic influenza vaccines including monovalent pandemic influenza vaccines.
• Previous vaccination against a FV disease at any time including yellow fever (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 vaccination and up to the 6 weeks following the vaccination
• Participation at the time of study enrollment (or in the 4 weeks preceding the study vaccination) or planned participation during the course of the study in another clinical study investigating a vaccine, drug, medical device, or medical procedure
• Deprived of freedom by an administrative or court order, or in an emergency setting, or hospitalized involuntarily
• 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 vaccine administration

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

Participants, Sponsor study staff, investigators and study staff, will remain blinded during the study conduct, except dedicated study staff preparing/administering the study interventions and who are not involved in the safety evaluation.