A Clinical Study to Investigate the Safety and Immunogenicity in Relation to Product Attributes of mRNA-1083 (Severe Acute Respiratory Syndrome Coronavirus 2 [SARS-CoV-2] and Influenza Vaccine)

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

• Investigator assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures.
• Participants of nonchildbearing potential may be enrolled in the study.
• Participants who could become pregnant: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy.
• 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.
• Have received an influenza vaccine in the previous season (since September 2023 for Part 1, since Sep 2024 for Part 2).

Exclusion criteria

• Reported diagnosis or condition that is associated with increased risk of severe influenza disease or complications, including individuals with chronic medical conditions.
• Reported history of congenital or acquired immunodeficiency, immunosuppressive condition, asplenia, or recurrent severe infections.
• 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.
• 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.
• 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.
• 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.
• Received a licensed seasonal influenza vaccine within 150 days prior to Day 1.
• Received a licensed/authorized SARS-CoV-2 vaccine within 90 days prior to Day 1.
• 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.