The purpose of this study is to evaluate the safety, reactogenicity, and immunogenicity in relation to the product attributes of mRNA-1083 vaccine when administered as a single intramuscular (IM) injection in adults ≥50 to <65 years of age.

The study will be comprised of 2 parts, Parts 1 and 2. In Part 1, participants will be randomized into 4 study groups (Lots A, B, C, and D). Upon completion of Part 1, participants will be randomized into 2 study groups (Lots A and E) in Part 2.

Inclusion Criteria:

1. Investigator assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures.
2. Participants of nonchildbearing potential may be enrolled in the study.
3. Participants who could become pregnant: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy.
4. Capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in the protocol.
5. Have received an influenza vaccine in the previous season (since September 2023 for Part 1, since Sep 2024 for Part 2).

Exclusion Criteria:

1. Reported diagnosis or condition that is associated with increased risk of severe influenza disease or complications, including individuals with chronic medical conditions.
2. Reported history of congenital or acquired immunodeficiency, immunosuppressive condition, asplenia, or recurrent severe infections.
3. History of anaphylaxis or severe hypersensitivity reaction requiring medical intervention after receipt of any of the following: mRNA vaccine or therapeutic; components of an mRNA vaccine or therapeutic; influenza vaccine; or components of an influenza vaccine, including egg protein.
4. Received corticosteroids at ≥10 milligrams (mg)/day of prednisone or equivalent for >14 days in total within 90 days prior to Day 1 or is anticipating the need for corticosteroids at any time during the study.
5. Received systemic immunosuppressive treatment, including long-acting biological therapies that affect immune responses (e.g., infliximab), within 180 days prior to Day 1 or plans to do so during the study.
6. Received or plans to receive any vaccine authorized or approved by a local health agency within 28 days prior to Day 1 or plans to do so within 28 days post study injection.
7. Received a licensed seasonal influenza vaccine within 150 days prior to Day 1.
8. Received a licensed/authorized SARS-CoV-2 vaccine within 90 days prior to Day 1.
9. Received any investigational influenza vaccine, investigational coronavirus disease 2019 (COVID-19) vaccine, or investigational combination vaccine for influenza and COVID-19 within 12 months prior to Day 1. Participants from mRNA-1083-P201 Part 1 will also be excluded from Part 2.

Other protocol-defined inclusion/exclusion criteria may apply.