The purpose of this study is to evaluate the safety and immunogenicity of a single intramuscular (IM) injection of up to 2 dose levels of Quadrivalent Influenza messenger ribonucleic acid (mRNA) Vaccine MRT5407 compared to an active control (QIV standard-dose (SD), QIV high-dose (HD) [adults ≥ 65 years of age only], or quadrivalent recombinant influenza vaccine (RIV4)) in adults 18 years of age and older.

Study duration is approximately 12 months.

Inclusion Criteria:

• Aged 18 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 12 weeks after study intervention administration.

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 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)
• Previous history of myocarditis, pericarditis, and / or myopericarditis
• Self-reported thrombocytopenia, contraindicating Intramuscular vaccination based on Investigator's judgment
• Bleeding disorder, or receipt of anticoagulants in the 3 weeks preceding inclusion, contraindicating Intramuscular vaccination based on Investigator's judgment
• Moderate or severe acute illness / infection (according to Investigator's judgment) or febrile illness (temperature ≥ 38.0°C [≥ 100.4°F]) on the day of vaccination. A prospective participant should not be included in the study until the condition has resolved or the febrile event has subsided
• Self-reported or documented seropositivity for human immunodeficiency virus, hepatitis B virus, or hepatitis C virus
• Receipt of any mRNA vaccine/product in the 2 months preceding study intervention administration

NOTE: The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.

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

This is a parallel-group prevention study with up to 5 arms that will be blinded to participants, investigators / sub-investigators, outcomes assessors, and laboratory personnel (with the exception of the sentinel safety portion of the study, which will be open-label). The Sponsor study staff will be unblinded (except laboratory testing personnel).