The purpose of this study is to learn about the safety, tolerability and immune effects of a pneumococcal vaccine in toddlers.

Participants will take part in this study for approximately 6 or 8 months depending on which group they are assigned to. During this time participants will have 2 clinic visits and 1 phone visit for the 1-Dose group, or 3 clinic visits and 1 phone visit for the 2-Dose group.

At these clinic visits, participants will be asked if any side effects were experienced. The participants will also have to give blood samples during some clinic visits.

Inclusion Criteria:

• Male or female toddlers ≥12 to ≤15 months of age at the time of consent.
• Healthy toddlers determined by clinical assessment, including medical history and clinical judgment, to be eligible for the study.
• Have received 3 infant doses of 20vPnC with the last dose administered >56 days before enrollment into the study. Documented confirmation of receipt will be collected prior to randomization.

Exclusion Criteria:

• History of severe adverse reaction associated with a vaccine and/or severe allergic reaction (eg, anaphylaxis).
• Major known congenital malformation or serious chronic disorder.
• Other acute or chronic medical or psychiatric condition or laboratory abnormality that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the participant inappropriate for entry into this study.
• Previous vaccination with any licensed or investigational pneumococcal vaccine (other than 20vPnC) or planned receipt during study participation.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.

Study is partially blinded as 2 of the 3 groups are blinded and 1 is unblinded.