Study Purpose: This study aims to test the safety and how well a new pneumococcal vaccine works for toddlers. Pneumococcal vaccines protect against bacteria that can cause ear, lung, and other infections.

Study Duration and Visits: Kids will be in the study for about 6 to 8 months. If they are in the 1-Dose group, they will have 2 clinic visits and 1 phone call. The 2-Dose group will have 3 clinic visits and 1 phone call. During visits, parents will be asked about any side effects, and some blood samples will be collected.

Eligibility: Toddlers between 12 to 15 months who have already received three doses of the 20vPnC vaccine as infants can participate. Kids with severe allergy history or certain medical conditions cannot join.

• The study lasts for 6 to 8 months with a few clinic visits.
• Kids need to have had three doses of the 20vPnC vaccine as infants.
• Blood samples will be collected, and parents will report any side effects.