The purpose of this study is to evaluate the safety and immunogenicity of a single intramuscular (IM) injection of different formulations of Quadrivalent Influenza Vaccine (QIV) messenger ribonucleic acid (mRNA) (MRT5421, MRT5424, and MRT5429) 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 per participant is approximately 12 months.

• Treatment duration: 1 injection of one of the 7 QIV mRNA or one of the controls
• Dose escalation with sequential enrollment

Inclusion Criteria: - Aged from 18 years on the day of inclusion or aged from 21 years on the days of inclusion, depending on the countries.

• 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.

• A female participant of childbearing potential must have a negative highly sensitive pregnancy test (urine or serum as required by local regulation) within 8 hours before the 1st dose of study intervention 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)

• Known systemic hypersensitivity to any of the study intervention components (eg, polyethylene glycol, polysorbate); history of a life-threatening reaction to the study interventions used in the study or to a product containing any of the same substances; any allergic reaction (eg, anaphylaxis) after administration of mRNA vaccine

• Previous history of myocarditis, pericarditis, and / or myopericarditis

• Known history of previous episodes of Gillian-Barre Syndrome (GBS), neuritis (including Bell's palsy), convulsions, encephalitis, transverse myelitis, and vasculitis

• Participants with an ECG that is consistent with possible myocarditis or pericarditis or, in the opinion of the investigator, demonstrates clinically relevant abnormalities that may affect participant safety or study results

• 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

• 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'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

• Participant who had acute infection symptoms or a positive SARS-CoV-2 RT-PCR or antigen test in the past 10 days prior to the 1st visit (V01)

• Receipt of any vaccine in the 4 weeks preceding study intervention administration or planned receipt of any vaccine in the 4 weeks following study intervention administration

• Receipt of immune globulins, blood or blood-derived products in the past 3 months

• Previous vaccination against influenza in the previous 6 months with an investigational or marketed vaccine

• 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

Modified double-blind (Participants; Sites except for those preparing/administering study intervention; Sponsor's except Sponsor unblinded internal safety review committee)