A Study to Evaluate the Safety and Efficacy of mRNA-1345 Vaccine Targeting Respiratory Syncytial Virus (RSV) in Adults ≥60 Years of Age

Key Inclusion Criteria (Part A): * Adults ≥ 60 years of age who are primarily responsible for self-care and activities of daily living. Participants may have one or more chronic medical diagnoses (including chronic heart failure \[CHF\] and chronic obstructive pulmonary disease \[COPD\]), but should be medically stable * Body mass index from ≥18 kilograms (kg)/square meter (m\^2) to ≤35 kg/m\^2 Key Inclusion Criteria (Part B): * Randomized to and were subsequently vaccinated with the mRNA-1345 study injection in Part A at least 21 months prior to Screening. Key Exclusion Criteria (Part A): * Participation in another clinical research study where participant has received an investigational product (drug/biologic/device) within 6 months before the planned date of the Day 1 study injection. * Current participation in research involving receipt of any investigational RSV product * History of a serious reaction to any prior vaccination or Guillain-Barré syndrome within 6 weeks of any prior influenza immunization. * Received or plans to receive any non-study vaccine within 28 days before or after the Day 1 study injection. Key Exclusion Criteria (Part B): * Participation in another clinical research study where participant has received an investigational product (drug/biologic/device) within 6 months before the planned date of BD study injection (BD Day 1). * History of a serious reaction to any prior vaccination, or Guillain-Barré syndrome within 6 weeks of any prior influenza immunization. * Received or plans to receive any non-study vaccine (including authorized or approved vaccines for the prevention of coronavirus disease 2019 (COVID-19) regardless of type of vaccine) within 14 days before or after the BD Day 1 study injection. * Received or plans to receive any commercial RSV vaccination at any time prior to BD study injection (BD Day 1) or during the substudy. Other inclusion and/or exclusion criteria may apply.