This study is testing a new vaccine called **VAX-31**. The goal is to see if it is safe, easy to handle (tolerable), and how well it works in people 50 years and older. The study also compares VAX-31 to other vaccines, **PCV21** and **PCV20**, which are approved for use. **PCV** stands for pneumococcal conjugate vaccine, which protects against diseases caused by pneumococcal bacteria, like pneumonia. People aged 18 and older can join this study if they're healthy or have stable health conditions. Before joining, participants must agree to several requirements, like giving blood samples and using a mobile device to fill out an electronic diary (eDiary). They should not have had certain vaccines recently or have medical conditions like a bleeding disorder or an infection like HIV. Pregnant or breastfeeding women cannot participate.

• The study requires regular follow-ups and diary entries.
• You need to be in good health or have stable health conditions.
• Some people, like those with certain infections, cannot join.