Study to Investigate the Immunogenicity, Reactogenicity, and Safety of mRNA-1083 Vaccine (SARS-CoV-2 [COVID-19] and Influenza) in Adults ≥50 Years of Age

Key Inclusion Criteria:

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

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

• Not pregnant or breast/chestfeeding, and is using a contraceptive method thatis highly effective for at least 90 days after the study interventionadministration and agrees not to donate eggs (ova, oocytes) for the purpose ofreproduction during this period.

• Have a negative highly sensitive pregnancy test (urine) as required by localregulations at Screening/Day 1 Visit, before the study intervention.

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

• Refrain from donating sperm.

• Either be abstinent from reproductive sexual intercourse as their preferred andusual lifestyle (abstinent on a long term and persistent basis) and agree toremain abstinent or must agree to use an external condom with a person whocould become pregnant partner use of an additional highly effectivecontraceptive method with a failure rate of <1% per year when having sexualintercourse.

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

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

Key Exclusion Criteria:

• History of a diagnosis or condition that, in the judgment of the Investigator, isclinically 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 ofdrug or alcohol abuse, that, in the opinion of the Investigator, might poseadditional risk due to participation in the study or could interfere with theinterpretation 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 intotal within 90 days prior to Day 1 or is anticipating the need for corticosteroidsat any time during the study. Inhaled nasal and topical steroids are allowed.

• Received systemic immunosuppressive treatment, including long-acting biologicaltherapies that affect immune responses (for example, infliximab), within 180 daysprior to Day 1 or plans to do so during the study.

• Received or plans to receive any vaccine authorized or approved by a local healthagency ≤28 days prior to study injections or plans to receive a vaccine authorizedor 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 daysprior to Day 1.

• Has had close contact with someone with laboratory-confirmed influenza infection orwith someone who has been treated with antiviral therapies for influenza (forexample, Tamiflu) within 5 days prior to Day 1

• Has had close contact to someone with COVID-19 as defined by the Centers for DiseaseControl 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 theScreening Visit/Day 1 or plans to donate blood products during the study.

• Reported history of anaphylaxis or severe hypersensitivity reaction after receipt ofany 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.