A Study to Investigate the Immunogenicity and Safety of mRNA-1345 Vaccine Targeting Respiratory Syncytial Virus (RSV) in High-risk Adults

Key Inclusion Criteria: Part A: * Adults ≥18 to \<60 years of age at the time of consent who are primarily responsible for self-care and activities of daily living. * Documented confirmation by a physician of the diagnosis of at least one of the following conditions: 1. Coronary artery disease and/or congestive heart failure. 2. Chronic lung disease (for example, including but not limited to chronic obstructive pulmonary disease or persistent asthma). 3. Stable type 1 or type 2 diabetes mellitus controlled with at least 1 medication started 90 days or more prior to Day 1. Part B: * Adults ≥18 years of age at the time of consent who are primarily responsible for self-care and activities of daily living. * Recipient of an SOT (kidney, liver, or lung transplant) ≥180 days prior to day of consent and receiving chronic immunosuppressive therapy for the prevention of allograft rejection. Parts A and B: \- Able to comply with study requirements. Key Exclusion Criteria: Part A: * Participation in an another clinical research trial where the participant has received an investigational product 180 days prior to Day 1. Participation in an RSV trial at any time prior to Day 1 is exclusionary. * History of a diagnosis or condition, that, in the judgment of the Investigator, is clinically unstable or may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to study procedures. * Reported history of anaphylaxis or severe hypersensitivity reaction after receipt of the mRNA-1345 injection or any components of the mRNA-1345 injection. * Chronic administration (defined as more than 14 continuous days) of immunosuppressants or other immune-modifying drugs within 180 days prior to Day 1. * History of myocarditis, pericarditis, or myopericarditis. Part B: * Previous treatment with alemtuzumab or rituximab within 2 years prior to Day 1. * Previous treatment with plasmapheresis within 30 days prior to Day 1. * A history of complications of immunosuppression. * A history of biopsy proven or clinically diagnosed rejection within 90 days prior to Day 1 or suspected active chronic rejection according to the Investigator's judgment. Note: Other protocol-defined inclusion/exclusion criteria apply