A Study to Evaluate the Safety, Reactogenicity, and Immunogenicity of mRNA-1010 in Healthy Adults

Inclusion Criteria: * Adults 18-50 years (Study Parts A, B, and C) and 65-80 years (Study Parts A and B only) of age at the time of consent (Screening Visit). * Part A only: Body mass index (BMI) of 18 kilograms (kg)/meter (m)\^2 to \< 40 kg/m\^2 at the Screening Visit. There will be no BMI requirement for inclusion in Part B and Part C. * A person of childbearing potential (POCBP): has a negative highly sensitive pregnancy test at the Screening Visit and before each administration of study intervention; must use a contraceptive method that is highly effective from at least 28 days prior to Day 1 (Baseline) to at least 3 months after the last study intervention; and is not currently pregnant or breastfeeding. Exclusion Criteria: * Acutely ill or febrile (temperature ≥ 38.0°Celsius (C)/100.4° Fahrenheit (F) hours before or at the D1 vaccination visit. Participants meeting this criterion may be rescheduled within the 28-day screening window. * 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. * Reported history of congenital or acquired immunodeficiency, immunocompromizing or immunosuppressive condition, asplenia, or history of recurrent severe infections. Certain immune-mediated conditions that are stable and well-controlled (for example, Hashimoto's thyroid disease) or that do not require systemic immunosuppressive therapy may be permitted at the discretion of the Investigator. * Dermatologic conditions that could affect local solicited AR assessments (tattoos, psoriatic patches or vitiligo affecting skin over the deltoid injection site area). * Has received systemic immunosuppressants (for glucocorticoids ≥ 10 mg/day of prednisone or equivalent) for \> 14 days in total within 180 days before vaccination visit (D1) 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. * Has received systemic immunoglobulins or long-acting biological therapies that may suppress or alter immune responses (for example, Infliximab®) or blood products within 90 days before the vaccination visit or plans to receive them during the study. * Has a 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. Other protocol-defined inclusion/exclusion criteria apply.