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.