The purpose of this study is to evaluate the safety and immunogenicity an investigational messenger ribonucleic acid (mRN)A vaccine for the prevention of lower respiratory tract disease (LRTD) caused by respiratory syncytial virus (RSV) and/or human metapneumovirus (hMPV) in older adults. A single intramuscular (IM) injection of 3 to 4 different doses of the RSV/hMPV mRNA vaccine candidate formulated with 2 different lipid nanoparticles (LNP) will be administered to healthy participants aged 18 to 49 years and 60 years and older. Treatment: * RSV/hMPV mRNA / LNP 1 at 3-4 different doses or, * RSV/hMPV mRNA / LNP 2 at 3-4 difference doses or, * RSV mRNA / LNP 1 at 1 dose or, * hMPV mRNA / LNP 1 at 1 dose
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AI-rewrites the medical criteria so a patient or caregiver can understand them. Always confirm with the trial site.
Presence of unsolicited systemic immediate adverse events (AEs)
Timeframe: Within 30 minutes after vaccination
Presence of solicited injection site or systemic reactions
Timeframe: Within 7 days after vaccination
Presence of unsolicited AEs
Timeframe: Within 28 days after vaccination
Presence of medically attended adverse events (MAAEs)
Timeframe: Throughout study (up to approximately 6 months)
Presence of serious adverse events (SAEs)
Timeframe: Throughout study (up to approximately 6 months)
Presence of adverse events of special interest (AESIs)
Timeframe: Throughout study (up to approximately 6 months)
Presence of out-of-range biological test results
Timeframe: Within 7 days after vaccination
RSV-A serum neutralizing antibody (nAb) titers at pre-vaccination (D01) and 28 days post- vaccination (D29) in the RSV/hMPV and monovalent RSV formulations
Timeframe: Day 1 and Day 29
RSV-B serum neutralizing antibody (nAb) titers at pre-vaccination (D01) and 28 days post- vaccination (D29) in the RSV/hMPV and monovalent RSV formulations
Timeframe: Day 1 and Day 29
hMPV- A serum neutralizing antibody (nAb) titers at pre-vaccination (D01) and 28 days post- vaccination (D29) in the RSV/hMPV and monovalent hMPV formulations
Timeframe: Day 1 and Day 29