The purpose of this study is to evaluate Immunogenicity and Safety of a High-Dose (HD) Influenza Vaccine (IV) in Adults 50 through 64 Years of Age.

The study duration will be approximately 180 days (+ 14 days).

Inclusion Criteria:

• Aged 50 to 64 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 study intervention administration until at least 4 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) at enrollment before the first dose of study intervention.

  • 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).
• Personal or family history of Guillain-Barré Syndrome (GBS).
• Known systemic hypersensitivity to any of the study intervention components (Section 1.1, Section 6.1), or history of a life-threatening reaction to the study intervention used in the study or to a product containing any of the same substances.
• Self-reported thrombocytopenia, contraindicating intramuscular injection, based on investigator's judgment.
• Bleeding disorder, or receipt of anticoagulants in the 3 weeks preceding inclusion, contraindicating intramuscular injection, based on investigator's judgment.
• Chronic illness that, in the opinion of the investigator, is at a stage where it might interfere with study conduct or completion.
• 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.
• 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 intervention administration or planned receipt of any vaccine prior to the second blood drawing (ie. approximately 4 weeks after the study intervention administration).
• Previous vaccination against influenza in the previous 6 months with an investigational or marketed vaccine.
• Receipt of immune globulins, blood, or blood-derived products in the past 3 months.
• Participation at the time of study enrollment (or in the 4 weeks preceding the first study intervention administration) or planned participation during the present study period in another clinical study investigating a vaccine, drug, medical device, or medical procedure.
• Deprived of freedom by an administrative or court order, or being 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.

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

Modified double-blind:

• Participants, Sponsor study staff*, investigators, and study site staff will not know which study vaccine is administered during the study conduct

• Dedicated study site staff who prepare/administer the study interventions and are not involved in the safety evaluation will know which study vaccine is administered.

  • Select sponsor staff will be unblinded after the code break for primary completion analysis.