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.