Substudy 01 - Safety and Immunogenicity of One Monovalent Modified mRNA Vaccine Encoding Influenza Hemagglutinin With LNP, in Adult Participants Aged 18 to 49 Years and 60 Years and Above

Inclusion Criteria:

• Aged 18 years and above on the day of inclusion

  *Aged 18 years to 49 years or 60 years and above on the day of inclusion (substudy 01)

• 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) at the screening
             visit.
          -  Inclusion Criteria to be Checked at Visit 1 (Day 1)
        Participants are eligible for the study only if all of the following criteria are met:
        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
        first dose of study intervention.
        Exclusion Criteria:
          -  Previous vaccination against influenza in the previous 6 months with an
             investigational or marketed vaccine
          -  Any screening laboratory parameter with laboratory abnormalities that are greater than
             Grade 1 or deemed clinically significant in the opinion of the Investigator
               -  OR, any screening Liver Function Test (ALT, AST, Bilirubin) > 1.2x Upper Limit of
                  Normal or any other screening laboratory parameter outside of the range of normal
                  limits for age and gender
          -  Positive test for human immunodeficiency virus (HIV) antigen and/or antibodies (Abs),
             hepatitis B (HB) virus surface antigen (HBsAg), hepatitis B core antibodies (HBcAb),
             or hepatitis C virus antibodies (HCV Abs)
          -  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 [PEG], 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
             COVID-19 vaccine
          -  Previous history of myocarditis, pericarditis, and/or myopericarditis
          -  Screening electrocardiogram (ECG) or troponin value that is consistent with probable
             or possible myocarditis, pericarditis, and/or myopericarditis or screening ECG that
             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
          -  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 study enrollment or planned receipt of
             any vaccine in the 4 weeks following study intervention administration
          -  Receipt of any mRNA vaccine/product in the 2 months preceding study enrollment or
             planned receipt of any mRNA vaccine/product within the 2 months following study
             intervention administration
          -  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 study
             enrollment or planned participation during the present study period in another
             clinical study investigating a vaccine, drug, medical device, or medical procedure
          -  Previous vaccination against influenza in the previous 6 months with an
             investigational or marketed vaccine
          -  Exclusion criteria to be checked at Visit 1 Day 1:
               -  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.
        The above information is not intended to contain all considerations relevant to a potential
        participation in a clinical trial.