This study is testing a new pneumococcal conjugate vaccine, called PG4, which helps protect against a type of bacteria that can cause serious infections like pneumonia. The study will check if this new vaccine is safe and works as well as the current one, 20vPnC. Participants will be divided into four groups to receive one of three different batches (lots) of the new PG4 vaccine or the current 20vPnC vaccine. The study will take about six months, and during this time, blood samples will be collected to see how the body reacts to the vaccines.

• Participation length: About 6 months.
• Participants: Adults aged 18-49 without a recent pneumococcal vaccine.
• Risks: People with severe allergies, certain medical conditions, and pregnant or breastfeeding women cannot join.