A Study of mRNA-1010 Compared With a Licensed Influenza Vaccine in Adults ≥50 Years of Age

Inclusion Criteria:

• Capable of giving signed informed consent, which includes compliance with therequirements and restrictions listed in the informed consent form (ICF) and in theprotocol.

• Participants who are assigned female at birth or can become pregnant are eligible toparticipate if:

• The participant is a person of nonchildbearing potential (PONCBP) or

• The participant is a person of childbearing potential (POCBP) who:

• Is not breast/chest feeding.

• Is using an acceptable contraceptive method at least 28 days prior to Day 1 (Baseline) to at least 90 days after Day 1 (Baseline).

• Has a negative highly sensitive pregnancy test (urine or serum as required by localregulation or institutional review board [IRB]/independent ethics committee [IEC])at the Screening Visit and before study intervention (if the Day 1 [Baseline] Visitis not on the same day as the Screening Visit).

Exclusion Criteria:

• Acutely ill or febrile (temperature ≥38.0 degrees Celsius (℃) [100.4° Fahrenheit [F]]) within 72 hours prior to Day 1 (Baseline).

• Close contact with someone with laboratory-confirmed influenza infection or withsomeone who has been treated with antiviral therapies for influenza (for example,Tamiflu®/oseltamivir) within 5 days prior to Day 1 (Baseline).

• 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.

• Reported history of congenital or acquired immunodeficiency, immunosuppressivecondition, asplenia, or recurrent severe infections disease.

• Tested positive for influenza by local health authority-approved testing methodswithin 180 days prior to Day 1 (Baseline).

• History of anaphylaxis or severe hypersensitivity reaction requiring medicalintervention after receipt of any of the following: mRNA vaccine or therapeutic;components of an mRNA vaccine or therapeutic; influenza vaccine; or components of aninfluenza vaccine, including egg protein.

• Malignancy within 2 years prior to Day 1 (Baseline) (adequately treated basal cellcarcinoma and squamous cell carcinoma are allowed).

• Received corticosteroids at ≥10 milligram (mg)/day of prednisone or equivalent for >14 days in total within 90 days prior to Day 1 (Baseline) or is anticipating theneed for corticosteroids at any time during the study.

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

• Treated with antiviral therapies for influenza (for example, Tamiflu) within 180days prior to Day 1 (Baseline).

• Received any vaccine authorized or approved by local health agency within 28 daysprior to Day 1 (Baseline) or plans to do so within 14 days after Day 1 (Baseline).

• Received a licensed seasonal influenza vaccine within 180 days prior to Day 1 (Baseline) or plans to do so (outside of this study) at any time during the study.

• Received an investigational seasonal influenza vaccine within 1 year prior to Day 1 (Baseline). Note: participants from mRNA-1010-P304 Season 1 are NOT eligible to reenroll into Season 2.

• Participated in a clinical study with investigational treatment within 90 days priorto Day 1 (Baseline) based on the medical history interview or plans to do so whileparticipating in this study.

• Is working or has worked as study personnel or is an immediate family member orhouse member of study personnel, study staff, or Sponsor personnel.