A Study of mRNA-1010 Seasonal Influenza Vaccine in Adults 50 Years Old and Older

Inclusion Criteria

• Investigator has assessed that the participant understands and is willing and physically able to comply with protocol mandated follow-up, including all procedures.
• For female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose on Day 1, and agreement to continue adequate contraception through 90 days following vaccine administration.

Exclusion Criteria

• Participant 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 the past 5 days prior to the Screening visit.
• Participant has had close contact to someone with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection or COVID-19 as defined by the US CDC or has had a positive SARS-CoV-2 test in the past 10 days prior to the Screening visit.
• Participant is acutely ill or febrile (temperature ≥38.0℃elcius [100.4°Fahrenheit]) 72 hours prior to or at the Screening visit or Day 1. Participants meeting this criterion may be rescheduled within the 28-day screening window.
• Participant has a history of a diagnosis or condition that, in the judgment of the investigator, is clinically unstable or may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to study procedures.
• Reported history of congenital or acquired immunodeficiency, immunocompromising/ immunosuppressive condition, asplenia, or recurrent severe infections.
• 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.
• Participant has received systemic immunosuppressant drugs for >14 days in total within 180 days prior to the Screening visit (for glucocorticosteroids, ≥10 milligrams (mg)/day of prednisone or equivalent) or is anticipating the need for systemic immunosuppressive treatment at any time during participation in the study. Inhaled, nasal, and topical steroids are allowed.
• Participant has received any vaccine authorized or approved by local health agency ≤28 days prior to study intervention (Day 1) or plans to receive a vaccine authorized or approved by local health agency within 28 days before or after the study intervention.
• Participant is unaware whether they have received an influenza vaccine in the previous influenza season.
• Participant received a seasonal influenza vaccine or any other investigational influenza vaccine within 180 days prior to Day 1
• Participant has tested positive for influenza by local health authority-approved testing methods within 180 days prior to Day 1.
• Participant has donated ≥450 milliliters (mL) of blood products within 28 days prior to the Screening visit or plans to donate blood products during the study.

Note: Other inclusion and exclusion criteria may apply.