The purpose of this study is to learn about the safety and tolerability of a pneumococcal vaccine in adults.

Participants will receive either:

• an experimental PG4 vaccine
• a PG4 vaccine comparator
• a standard 20vPnC vaccine comparator
• placebo. A placebo does not have any medicine in it but looks just like the study medicine.

Participants will take part in this study for up to 18 months depending on which group they are assigned to. During this time, the participants will receive up to two doses of study vaccine or comparator and take part in follow-up visits.

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

Key Inclusion Criteria:

• Healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study.
• Participants either with no history of ever receiving a pneumococcal vaccine or who have received a pneumococcal vaccine more than 5 years prior to vaccination in this study

Key Exclusion Criteria:

• Participants with a history of microbiologically proven invasive disease caused by S pneumoniae.

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.