The purpose of the study is to evaluate the immunogenicity, reactogenicity, and safety of mRNA-1083 in adults 50 years of age and older in participating countries (Japan, Taiwan, and South Korea).

Key Inclusion Criteria:

• Medically stable adults ≥50 years of age at the time of consent (Screening/Day 1 Visit).

• A participant who could become pregnant is eligible to participate if the following is met:

  • Not pregnant or breast/chestfeeding, and is using a contraceptive method that is highly effective for at least 90 days after the study intervention administration and agrees not to donate eggs (ova, oocytes) for the purpose of reproduction during this period.
  • Have a negative highly sensitive pregnancy test (urine) as required by local regulations at Screening/Day 1 Visit, before the study intervention.

• Participants who can produce sperm are eligible to participate if they agree to the following for at least 90 days after the study intervention administration:

  • Refrain from donating sperm.
  • Either be abstinent from reproductive sexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis) and agree to remain abstinent or must agree to use an external condom with a person who could become pregnant partner use of an additional highly effective contraceptive method with a failure rate of <1% per year when having sexual intercourse.

• Agree to use an external condom when engaging in any activity that allows for passage of ejaculate to another person.

• Received ≥2 COVID-19 vaccines and the last COVID-19 vaccine was ≥150 days prior to Day 1

Key Exclusion Criteria:

• History of a diagnosis or condition that, in the judgment of the Investigator, is clinically unstable or may affect participant safety, assessment of study endpoints, assessment of immune response, or adherence to study procedures.
• Any medical, psychiatric, or occupational condition, including reported history of drug or alcohol abuse, that, in the opinion of the Investigator, might pose additional risk due to participation in the study or could interfere with the interpretation of study results.
• Known history of SARS-CoV-2 infection within 150 days prior to Day 1.
• Tested positive for influenza by local health authority-approved testing methods ≤150 days prior to Day 1.
• Received corticosteroids at ≥10 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. Inhaled nasal and topical steroids are allowed.
• Received systemic immunosuppressive treatment, including long-acting biological therapies that affect immune responses (for example, 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 ≤28 days prior to study injections or plans to receive a vaccine authorized or approved by a local health agency within 28 days after the study injections.
• Received a seasonal influenza vaccine ≤150 days prior to Day 1.
• Treated with antiviral therapies for influenza (for example, Tamiflu®) ≤150 days prior to Day 1.
• Has had close contact with someone with laboratory-confirmed influenza infection or with someone who has been treated with antiviral therapies for influenza (for example, Tamiflu) within 5 days prior to Day 1
• Has had close contact to someone with COVID-19 as defined by the Centers for Disease Control and Prevention (CDC) in the past 10 days prior to Day 1.
• Has donated ≥450 milliliters (mL) of blood products within 28 days prior to the Screening Visit/Day 1 or plans to donate blood products during the study.
• Reported history of anaphylaxis or severe hypersensitivity reaction after receipt of any mRNA or influenza vaccines or any components of the mRNA or influenza vaccines, including egg protein.

Note: Other protocol-defined inclusion/exclusion criteria may apply.