Study on an Investigational Yellow Fever Vaccine Compared With YF-VAX in Adults in the USA

The primary objective of the study is to demonstrate the non-inferiority of the antibody response in terms of seroconversion rate 28 days after vaccine administration of one dose of yellow fever vaccine (vYF) compared to the antibody response after one dose of the YF-VAX control vaccine in yellow fever naïve participants.

The secondary objectives of the study are:

* To describe the immune response to yellow fever in both vaccine groups before and after vYF or YF-VAX administration.
* To describe the safety profile of vYF vaccine in comparison to the safety profile of the control YF-VAX.
* To describe the biosafety profile of vYF in comparison to the biosafety profile of the control YF-VAX.

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

Eligibility Criteria:
Inclusion Criteria:

* Aged 18 years 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 or abstinence from at least 4 weeks prior to vaccination until at least 4 weeks after vaccination. A 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 will be repeated on Day 29 to confirm the participant is still not pregnant within the 28 days of vaccine administration.

* Informed consent form has been signed and dated.
* Able to attend all scheduled visits and to comply with all study procedures.

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 flavivirus 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 human immunodeficiency virus infection.
* Known history of hepatitis B or hepatitis C seropositivity
* 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 ≥ 100.4°F). 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 up to the 6 weeks following the vaccination
* 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 except for influenza vaccination, which may be received at least 2 weeks before study vaccines. This exception includes monovalent pandemic influenza vaccines and multivalent influenza vaccines.
* Previous vaccination against a flavivirus 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.
* Participation at the time of study enrollment (or in the 4 weeks preceding the study vaccination) or planned participation during the first year 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 year is permitted (starting the first day of Year 2, and onwards), assuming it does not exclude participation in this study
* 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.
* 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.