This is a Phase 3, blinded clinical trial evaluating the safety profile of an updated pneumococcal conjugate vaccine (PCV) in infants and toddlers. Participants will be randomly assigned to receive either the currently recommended Prevnar 20 regimen or the investigational PCV21 regimen. Prevnar 20 is formulated to protect against 20 strains of pneumococcal bacteria, while the investigational PCV21 is designed to provide coverage against an additional strain. Participation would include receiving the 4 doses of pneumococcal vaccine at 2 months, 4 months, 6 months, and 12-15 months of age.