A Study of Modified mRNA Vaccines in Healthy Adults

Inclusion Criteria: * Adults 18 to 75 years of age at the time of consent (screening visit) who, in the opinion of the investigator, are in good health based on review of medical history and physical examination performed at screening. For mRNA-1647 study arms, adults 18 to \<50 years of age at the time of consent (screening visit) who, in the opinion of the investigator, are in good health based on review of medical history and physical examination performed at screening. * Body mass index (BMI) of 18 kilograms (kg)/square meter (m\^2) to 35 kg/m\^2 (inclusive) at the screening visit. Exclusion Criteria: * Participant has had close contact to someone with confirmed SARS-CoV-2, respiratory syncytial virus (RSV), or influenza infection in the past 14 days prior to the screening visit. * Severe allergic reaction to any component of the FLUAD vaccine (active comparator), including egg protein, or after a previous dose of any influenza vaccine. * Participant has received systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months prior to screening (to include any systemic corticosteroids) or is anticipating the need for immunosuppressive treatment at any time during participation in the study. * Participant plans to receive any licensed or authorized vaccine, including COVID-19 influenza vaccines, within 28 days before or after any study injection. * Participant has received a NH 2021 - 2022 or 2022-2023 seasonal influenza vaccine or any other…