This is a study for a new vaccine called BLB-201, which helps protect against a virus known as RSV (Respiratory Syncytial Virus). RSV can cause breathing problems in young children. The vaccine is given as a spray into the nose. Children between 8 months and 59 months old can take part in this study. Some children will get the vaccine, and some will get a placebo, which is like a fake vaccine that helps researchers compare results. **Key Points:** - **Study Duration:** Participants will need to be available for the entire study period, which includes one or two doses of the vaccine. - **Eligibility:** Children must be generally healthy and have received their regular vaccines. - **Compensation:** Not mentioned, but families might consider potential benefits of vaccine protection. Some children cannot join, like those with certain health issues or who have had recent infections. The study makes sure it's safe by checking health conditions and past medical history. If you’re interested, talk to the study team for more details.