This study evaluates the **safety and effectiveness** of a new vaccine called PCV21, in comparison to an existing vaccine, Prevnar 20. The study is for healthy infants, toddlers, children, and adolescents. **PCV21** is a 21-valent pneumococcal conjugate vaccine, which means it helps protect against 21 types of bacteria that can cause serious infections like pneumonia. - **Participation Length**: Infants will participate for about 9 to 11 months, toddlers for 8 to 9 months, and children/adolescents for 6 months. - **Eligibility**: Participants must be healthy, aged 7 months to 17 years, and meet specific health criteria. Female adolescents must not be pregnant and should use birth control if needed. - **Exclusions**: Anyone with certain medical conditions, prior relevant vaccinations, or recent vaccine or drug use may not participate. Participants and their families need to attend all scheduled visits and follow study procedures. This study helps scientists ensure that the new vaccine is safe and works well for kids. Always consult with your doctor to understand if this study is right for you.