A Safety, Reactogenicity, and Immunogenicity Study of mRNA-1045 (Influenza and Respiratory Syncytial Virus [RSV]) or mRNA-1230 (Influenza, RSV, and Severe Acute Respiratory Syndrome Coronavirus 2 [SARS-CoV-2]) Vaccine in Adults 50 to 75 Years Old

Inclusion Criteria: * Investigator assessment that participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. * Body mass index of 18 to 35 kilograms/square meter (kg/m\^2) (inclusive) at the Screening Visit(s). * 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 or abstinence through 3 months following the vaccination, and not currently breastfeeding. * Fully vaccinated for COVID-19 with an approved primary series according to the locally authorized or approved regimen. The most recent COVID-19 vaccine (primary series or booster) must be ≥120 days before (or less per local guidance) Day 1. Exclusion Criteria: * Acutely ill or febrile (temperature ≥38.0°Celsius/\[100.4°Fahrenheit\]) 72 hours before or at the Screening Visit or Day 1. Participants meeting this criterion may be rescheduled within the 28-day screening window for re-evaluation and will retain their initially assigned participant number. * Any medical, psychiatric, or occupational condition, including reported history of drug or alcohol 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. * Has received systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months before screening (for corticosteroids ≥10 milligrams (mg)/day of prednisone or equivalent) or is anticipating the need for immunosuppressive treatment at any time during participation in the study. * Received or plans to receive any vaccine authorized or approved by a local health agency ≤28 days before study injections (Day 1) or within 28 days after the study injection. * Received a licensed seasonal influenza vaccine or any other investigational influenza or RSV vaccine within ≤180 days before Day 1. * Tested positive for influenza or RSV by local health authority-approved testing methods within ≤180 days before Day 1. * Significant exposure to someone with SARS-CoV-2 infection or COVID-19 in the past 14 days, as defined by the United States Center for Disease Control (CDC) or the European Centre for Disease Prevention and Control as a high risk (close contact) of a COVID-19 case or known history of SARS-CoV-2 infection within the past 90 days before Day 1. * Donated ≥450 mL of blood products within 28 days before the Screening Visit or plans to donate blood products during the study. Note: Other inclusion and exclusion criteria may apply.