A Study to Evaluate the Immunogenicity and Safety of 13-valent Pneumococcal Conjugate Vaccine (PCV13i) in Healthy Infants Aged 2 Months (Minimum 6 Weeks)

This is a Phase 3 randomized, observation-blinded, active-controlled, parallel-group clinical trial designed to evaluate the immunogenicity, safety, and functional antibody response of the experimental vaccine versus the control vaccine in healthy Thailand infants vaccinated at a 2+1 schedule (2 months, 4 months and 12-15 months). The trial will enroll approximately 600 healthy infants aged 2 months (at least 6 weeks) who will be randomly assigned in a 1:1 ratio to receive either the experimental or control vaccine, with 100 in each group (200 in total) randomized to subgroups and subject to additional immunogenicity assessments. All participants will be evaluated for solicited adverse events for 7 days and unsolicited adverse events for 30 days post each vaccination. Immunogenicity evaluation will be performed in all participants at baseline and post the booster dose, while the sub-cohort participants will be evaluated for post primary series immunogenicity additionally.

Inclusion Criteria: * Healthy infants with stable clinical conditions aged 2 months (42-90 days) at the time of screening, based on medical history and clinical assessment by the investigator. Infants will be eligible starting from the day they turn 6 weeks of age. * Infant's parent or legal guardian must be able and willing to provide informed consent for the infant's participation in the study. * Participants and their parent or legal guardian must demonstrate the ability to comply with all trial procedures and be available for the entire follow-up duration. * The infant's parent or legal guardian must have an easily identifiable and stable place of residence within the study area, be available for the duration of trial participation, and have access to a reliable means of telephone contact for communication with the study team. Exclusion Criteria for the first dose: * Infants born at \