**Study of the Safety of a 21-valent Pneumococcal Conjugate Vaccine in Healthy Infants** This study looks at how safe a new vaccine, **PCV21**, is for babies around 2 months old. A vaccine helps protect us from getting sick. Babies in this study will either get the PCV21 vaccine or another vaccine called **20vPCV**. The study lasts about 19 months, and babies will get vaccines at ages 2, 4, 6, and between 12 to 15 months. There will be 6 visits to the doctor during the study. To join, babies must be healthy and born full-term or a bit early but healthy. Babies can't join if they have certain health issues or have had some vaccines before. **Key Points:** - **Duration:** The study takes about 19 months with 6 visits. - **Vaccinations:** Babies get vaccines at 2, 4, 6, and 12-15 months of age. - **Eligibility:** Babies must be healthy and meet specific criteria to join. This study helps doctors learn more about keeping babies healthy with vaccines.