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

The purpose of this study is to learn about the safety of a new pneumococcal vaccine and how the new pneumococcal vaccine helps to fight against germs in infants when compared to the pneumococcal vaccines that are currently in use, 20vPnC (Prevnar 20®) or another licensed pneumococcal vaccine.

To ensure that the new vaccine (PG4) stays stable, it is placed in a liquid mixture of sterile water and other substances (a solution). This study will also test if there is a difference in the safety and immune effects of the new pneumococcal vaccine when it is one type of solution compared to when it is in a different type of solution.

The immune response is how the body's cells; tissues and organs work together to protect the body from infection. Blood samples will be used to measure the amount of antibodies produced after the vaccination. Antibodies are proteins that protect you when an unwanted germ enters the body. This will help understand how well the new pneumococcal vaccine works. This vaccine can possibly provide protection against pneumococcal disease. Pneumococcal disease includes a variety of infections caused by a specific germ, Streptococcus pneumoniae.

This study is seeking participants who are:

* male or female infants who are 2 months of age, * infants born at 36 weeks (about 8 and a half months) of pregnancy or later; and, * said to be healthy by the study doctor

There are four groups in this study. All participants will be assigned to one of the four groups. All study vaccines will be given as a single shot into the left thigh muscle. Participants in the three groups will have 3 blood samples collected during the 1 and a half years they are in the study.

The first 400 participants who enter the study will be assigned to either Group 1 or Group 2. Half the participants in Group 1 and half the participants in Group 2 will receive 4 doses at 2, 4, 6, and 12 to 15 months of age of PG4 mixed in the first solution. The other half of the participants in Groups 1 and 2 will receive 4 doses of 20vPnC (Prevnar 20®) at 2, 4, 6, and 12 to 15 months of age. The main difference between Groups 1 and 2 is that participants in Group 2 will have the first blood sample collected at an earlier time than those in Group 1.

Once 400 participants have been assigned to Groups 1 and 2 then 100 new participants will be assigned to Group 3. Half the participants in Group 3 will receive PG4 in the second solution at 2, 4, 6, and 12 to 15 months of age. The other half of the participants in Groups 3 will receive 4 doses of 20vPnC (Prevnar 20®) at 2, 4, 6, and 12 to 15 months of age.

Once the 100 participants have been assigned to Group 3 then 300 new participants will be assigned to Group 4. Half the participants in Group 4 will receive PG4 in the first solution at 2, 4, 6, and 12 to 15 months of age. The other half of the participants in Group 4 will receive 4 doses of a licensed pneumococcal comparator vaccine at 2, 4, 6, and 12 to 15 months of age.

Participants will take part in this study for about 16 to 19 months (about 1 and a half years). During this time, participants will have 6 study clinic visits and 1 to 2 phone calls. At these study clinic visits, parent(s) or legal guardian(s) will be asked if the participant experienced any side effects. A side effect is an unintentional or unexpected reaction to a vaccine.