This study is testing a new vaccine for two viruses: **RSV (Respiratory Syncytial Virus)** and **HMPV (Human Metapneumovirus)**. These viruses can cause lung infections, which are especially risky for older adults. The study includes 2400 people aged 60 and above, and it will last about one year for each person. There are already vaccines for RSV, but not for HMPV, which is why this new vaccine is important. Participants will be placed into one of four groups and could receive different vaccine doses or a placebo (an inactive substance). It's a "randomized, observer-blind" study, meaning neither participants nor the researchers know who gets the actual vaccine or the placebo, to make the results fair. **Key Points:** - **Duration:** The study lasts around 12 months per participant. - **Eligibility:** Must be 60+ years old and meet specific health criteria. - **Risks:** Excludes those with certain health conditions or recent vaccinations. Joining this study could help develop a new vaccine for older adults, but it's important to consider your health and the study's rules before participating.