A Study of mRNA-1011.1, mRNA-1011.2, and mRNA-1012.1 Candidate Seasonal Influenza Vaccines in Healthy Adults

Key Inclusion Criteria: Body mass index of 18 kilograms (kg)/square meter (m^2) to 35 kg/m^2 (inclusive) at the Screening Visit. 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 Day 1, agreement to continue adequate contraception through 3 months following vaccine administration, and not currently breastfeeding. Key Exclusion Criteria: Participant is acutely ill or febrile (temperature ≥38.0 degrees Celsius [°C]/100.4 degrees Fahrenheit [°F]) 72 hours prior to or at the Screening Visit or Day 1. Any medical, psychiatric, or occupational condition, including reported history of substance 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. Participant has received systemic immunosuppressants for >14 days in total within 180 days prior to the Randomization Visit (for glucocorticosteroids ≥10 milligrams [mg]/day of prednisone equivalent) or is anticipating the need for systemic immunosuppressive treatment at any time during participation in the study (including intra-articular steroid injections). Inhaled, nasal, and topical steroids are allowed. Participant has received or plans to receive any licensed or authorized vaccine, including COVID-19 vaccines, ≤28 days prior to the study injection (Day 1) or plans to receive a licensed or authorized vaccine within 28 days after the study injection. Participant has received a seasonal influenza vaccine or any other influenza vaccine within 180 days prior to the Randomization Visit. Participant tested positive for influenza by local health authority-approved testing methods within 180 days prior to the Randomization Visit. Participant has had close contact to someone with or been diagnosed themselves with respiratory syncytial virus or SARS-CoV-2 infection as defined by the Centers for Disease Control and Prevention (CDC) in the past 10 days prior to the Randomization Visit. Participant has donated ≥450 milliliters (mL) of blood products within 28 days prior to the Randomization Visit or plans to donate blood products during the study. Note: Other inclusion/exclusion criteria may apply.