A Study on the Efficacy of a Vaccine Against Influenza in Adults ≥65 Years of Age

Inclusion Criteria: 1. Participants who, in the opinion of the investigator, can and will comply with the requirements of the protocol independently or with the assistance of a caregiver. 2. Written or witnessed (physically or digitally) informed consent obtained from the participant/Legally acceptable representative \[LAR(s)\] of the participant prior to performance of any study-specific procedure. 3. Healthy participants or medically stable patients as established by medical history and clinical examination. Participants with chronic medical conditions with or without specific treatment are allowed to participate in this study if considered by the investigator as medically stable. A stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during 3 months before enrollment. 4. A male or female participant at least 65 YOA at the time of the study intervention administration. Exclusion Criteria: 1. History of test-confirmed influenza infection within 180 days prior to study intervention administration. 2. Any confirmed or suspected immunosuppressive or immunodeficient condition. HIV-infected individuals may be enrolled if they have been stable on antiretroviral therapy for the past 6 consecutive months, their CD4 cell count is ≥ 200/mm³ and their viral load has been undetectable. 3. Hypersensitivity to latex. 4. History of myocarditis or pericarditis considered causally associated with mRNA vaccine. 5. History of any reaction or hypersensitivity likely to be exacerbated by any component of the study intervention(s). 6. History of hypersensitivity or allergic reaction to any previous influenza vaccine. 7. History of hypersensitivity or allergic reaction to any previous mRNA vaccine. 8. History of Guillain-Barré syndrome (GBS) within 6 weeks of receiving any vaccine. 9. Significant underlying illness that in the opinion of the investigator would be expected to prevent completion of the study 10. Acute or unstable chronic conditions \[clinically significant pulmonary, cardiovascular, hepatic, metabolic/endocrine disorders, renal functional abnormality, or psychiatric conditions\], as determined by medical history and clinical examination. 11. Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant, due to participation in the clinical study. 12. Condition that in the judgment of the investigator would make intramuscular injection unsafe. 13. Administration of an influenza vaccine in the period starting 180 days before the study intervention administration or planned administration at any time during the study. 14. Planned administration/administration of any mRNA-based vaccine, live vaccine or adjuvanted protein-based vaccine in the period starting 30 days before the study intervention administration (Day -29 to Day 1) and ending 28 days after the study intervention administration (Day 1 to Day 28) or a non-replicating vaccine in the period starting 15 days before the study intervention administration (Day -14 to Day 1) and ending 14 days after the study intervention administration (Day 1 to Day 14). 15. Use of any investigational or non-registered product (drug, vaccine or invasive medical device in the country of enrollment) other than the study intervention during the period beginning 30 days before the dose of study intervention (Day -29 to Day 1), or their planned use during the study period. 16. Administration of immunoglobulins or other blood products or plasma derivatives during the period starting 90 days before the study intervention (Day -89 to Day 1) or planned administration during the study period. 17. Chronic administration of immune-modifying drugs (defined as more than 14 consecutive days in total) and/or planned use of long-acting immune-modifying treatments at any time up to the end of the study. * Up to 90 days prior to the study intervention administration: * For systemic corticosteroids: prednisone equivalent ≥20 mg/day for \>14 days. Inhaled, intra articular/intra-bursal, and topical steroids are allowed. * Long-acting immune-modifying drugs including among others immunotherapy (e.g., TNF-inhibitors), monoclonal antibodies, antitumoral medication. 18. Administration of antitumoral medication during the period starting 90 days before the study intervention (Day -89 to Day 1) or planned administration during the study period. 19. Administration of any vaccine in a clinical trial investigating an influenza mRNA vaccine in the period starting 1 year before the study intervention administration. 20. Concurrently participating in another clinical study at any time during the study period, in which the participant has been or will be exposed to an investigational or a non-investigational intervention (drug/invasive medical device). 21. For enrollment in Season 2: Previous participation (i.e., during previous season) in this clinical study. 22. Planned move during the study period that will prohibit participating in the study until study end. 23. Planned leave or holiday of 4 consecutive weeks or more during the influenza season covered by the study, that would prohibit the reporting of ILI cases and attendance to ILI visit. 24. Any study personnel or their immediate dependents, family, or household members.