A Study to Learn About How a New Pneumococcal Vaccine Works in Adults

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.