Study to Evaluate the Safety, Reactogenicity, and Immunogenicity of mRNA Vaccine Candidate Variations in Healthy Adults

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 assigned females at birth and 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 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 Day 1.
• 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, immunosuppressive condition or immune-mediated disease, asplenia, or recurrent severe infections.
• Participant has tested positive for influenza by local health authority-approved testing methods within 150 days prior to Day 1.
• Reported history of anaphylaxis or severe hypersensitivity reaction after receipt of mRNA vaccines or any components of the mRNA-1010 or influenza vaccines, including egg protein.
• Participant has received systemic immunosuppressants for >14 days in total within 180 days prior to Day 1 (for corticosteroids, ≥10 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. Intra-articular and epidural steroid injections are not allowed within 28 days before and/or after study intervention dosing.
• Participant has received any vaccine authorized or approved by local health agency ≤28 days prior to study intervention dosing (Day 1) or plans to receive a vaccine authorized or approved by local health agency within 28 days before or after study intervention dosing.
• Participant has received a licensed seasonal influenza vaccine within 5 months (150 days) prior to Day 1.
• Participant has participated in any investigational seasonal influenza vaccine study within12 months prior to Day 1.
• Participant is not aware whether they have received an influenza vaccine in the prior 12 months.
• Participant has donated ≥450 milliliters (mL) of blood products within 28 days prior to Day 1 or plans to donate blood products during the study.