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 influenza vaccine, or an experimental RSV or cytomegalovirus (CMV) vaccine, within 6 months prior to study Day 1, and/or has not completed a primary vaccination series for COVID-19. * Participant has received an authorized or approved COVID-19 vaccine within 4 months prior to Day 1, and/or has not completed a primary vaccination series for COVID-19. * Participant had a laboratory-confirmed infection with influenza or RSV within 6 months prior to Day 1, or SARS-CoV-2 within 4 months of Day 1. SARS-CoV-2 infection confirmed with self-administered, authorized or approved rapid antigen test is acceptable. * 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. * Participated in an interventional clinical study within 28 days prior to the screening visit based on the medical history interview or plans to do so while participating in this study until 12 months after their last study vaccination. Other inclusion and exclusion criteria may apply.