This study is a Phase 3, randomized, modified double-blind study which aims to measure whether PCV21 vaccine (investigational pneumococcal conjugate vaccine) is safe and can help the body to develop germ-fighting agents called antibodies (immunogenicity) compared with 15vPCV (Vaxneuvance, licensed pneumococcal conjugate vaccine) when they are administered with routine pediatric vaccines in infants aged from approximately 2 months (42 to 112 days). The study duration per participant will be up to approximately 20 months. The study vaccines (either PCV21 or 15-valent pneumococcal vaccines) will be administered at approximately 2, 4, and 11 to 15 months of age or at approximately 2, 3, 4, and 11 to 15 months of age (for preterm infants). Routine pediatric vaccines will be given at the same timepoints, as per local practice / recommendations. • There will be 5 (for full-term infants) or 6 (for preterm infants) study visits: * Full-term infants: Visit (V)01, V02 separated from V01 by 60 days, V03 separated from V02 by 30 days, V04 at 11 months of age until 15 months of age, V05 separated from V04 by 30 days. * Preterm infants: Visit (V)01, V02 separated from V01 by 30 days, V03 separated from V02 by 30 days, V04 separated from V03 by 30 days, V05 at 11 months of age until 15 months of age, V06 separated from V05 by 30 days.