Parts A and B both: - Adults ≥50 years of age on the day of the Randomization Visit (Day 1) who are primarily responsible for self-care and activities of daily living. Participants may have one or more chronic medical diagnoses, but should be medically stable as assessed by: Absence of changes in medical therapy within 1 month due to treatment failure or toxicity; Absence of medical events qualifying as SAEs within 1 month of the planned vaccination on Day 1; and absence of known, current, and life-limiting diagnoses which, in the opinion of the investigator, would make completion of the protocol unlikely. - Able to comply with study requirements, including access to transportation for study visits. Part B only: - Fully vaccinated for COVID-19 with an approved primary series according to the locally authorized or approved regimen. If the most recent COVID-19 vaccine was part of a primary series, it must be ≥ 150 days before (or less per local guidance) Day 1. If the most recent COVID-19 vaccine was a booster dose, it must be ≥ 120 days before (or less per local guidance) Day 1. Part C: - Participants at Part C study sites who have been enrolled in Part B (Groups 4 and 5) of this study; have immunogenicity blood sampling at Part B baseline and Day 29; completed the Day 211/end-of-study visits for Part B; were included in the per-protocol (PP) set; and received 1 dose of mRNA-1345 at least 12 months (but no later than 15 months) prior to the time of enrollment. - Able to comply with study requirements, including access to transportation for study visits.

Part A: - Participant has received or plans to receive any vaccine authorized or approved by a local health agency ≤28 days prior to study injections (Day 1) or plans to receive a vaccine authorized or approved by a local health agency within 28 days after the study injections. - Prior participation in research involving receipt of any investigational product (drug/biologic/device including any investigational RSV product) within 45 days before the planned date of the Day 1 study injection. - Participant has received a seasonal influenza vaccine or any other investigational influenza vaccine ≤180 days prior to the Randomization Visit (Day 1). - History of a serious reaction to any prior vaccination, or Guillain-Barré syndrome within 6 weeks of any prior influenza immunization. - 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. Part B: - Participant has received or plans to receive any vaccine authorized or approved by a local health agency ≤ 28 days prior to study injections (Day 1) or plans to receive a vaccine authorized or approved by a local health agency within 28 days after the study injections (with the exception of SARS-Cov-2 vaccination). - Prior participation in research involving receipt of any investigational product (drug/biologic/device with the exception of RSV investigation products) within 45 days before the planned date of the Day 1 study injection. - Prior receipt of any investigational/approved RSV product within 1 year of the Day 1 study injection. - Has known history of SARS-CoV-2 infection within 90 days prior to enrollment. Parts A and B both: - Participant had significant exposure to someone with SARS-CoV-2 infection or COVID-19 in the past 10 days, as defined by the United States (US) Centers for Disease Control and Prevention (CDC) as a close contact of someone who has had COVID-19. Part C: - Participation in another interventional clinical research study where participant has received an investigational product (drug/biologic/device) within 6 months before the planned date of the BD Day 1 study injection. Any prior receipt of an investigational or approved vaccine against RSV, except as part of mRNA-1345 Study P302 Part B, is exclusionary. - Participant has received or plans to receive any vaccine authorized or approved by a local health agency ≤28 days prior to the study injection (BD Day 1) or plans to receive a vaccine authorized or approved by a local health agency within 28 days after the study injections. - History of a serious reaction to any prior vaccination or Guillain-Barré syndrome 6 weeks after any prior influenza immunization. Other inclusion and/or exclusion criteria may apply.