Safety, Tolerability, and Immunogenicity Study of a 31-Valent Pneumococcal Conjugate Vaccine (VAX-31) in Adults

Inclusion Criteria: 1. Male or female aged 50 to 64 years (inclusive) for Stage 1, or 50 years and older (inclusive) for Stage 2 at the time of randomization into the study. 2. Able and willing to complete the informed consent process. 3. Available for clinical follow-up through the last study visit at 6 months after the study vaccination. 4. In good general health as determined by medical history, vital signs, physical examination, and clinical judgment of the Investigator. 5. Willing to have blood samples collected, stored indefinitely, and used for research purposes. 6. Able to provide proof of identity to the satisfaction of the study staff completing the enrollment process. 7. Women of childbearing potential, defined as premenopausal females capable of becoming pregnant, must have a negative urine pregnancy test immediately prior to randomization and agree to use acceptable contraception 8. Able to access and use a smartphone, tablet, computer, or other device connected to Wi-Fi or cellular network for completion of an electronic diary. Exclusion Criteria: 1. Previous pneumococcal disease (either confirmed or self-reported). 2. Previous receipt of a licensed or investigational pneumococcal vaccine. 3. Receipt of any investigational study product within 30 days prior to enrollment into the study, currently participating in another interventional investigational study, or having plans to receive another investigational product(s) while on study. 4. Planned or actual administration of any licensed vaccine during the period starting 30 days before enrollment into the study through Month 1. 5. Physical examination indicating any clinically significant medical condition. 6. Body Temperature \>38.0°C (\>100.4°F) or acute illness within 3 days prior to study vaccination (subject may be rescheduled). 7. Previous or existing diagnosis of HIV, Hepatitis B, or Hepatitis C. 8. History of severe allergic reaction with generalized urticaria, angioedema, or anaphylaxis. 9. Female who is pregnant, breastfeeding, or planning to become pregnant during study participation. 10. Bleeding disorder diagnosed by a doctor (e.g., factor deficiency, coagulopathy, or platelet disorder requiring special precautions) resulting in clinically significant bruising or bleeding difficulties with IM injections or blood draws. 11. Any other chronic or clinically significant medical condition that, in the opinion of the Investigator, would jeopardize the safety or rights of the subject or confound evaluation of the study vaccine. 12. Any medical, psychiatric, or social condition that in the judgment of the Investigator is a contraindication to protocol participation or impairs a subject's ability to give informed consent. 13. Received blood or blood product (including Immune Globulin IV) within 90 days prior to enrollment into the study. 14. Received systemic corticosteroids (except for inhaled, topical, intra-articular) for ≥14 consecutive days and has not completed treatment ≤30 days prior to enrollment into the study. 15. Receiving immunosuppressive therapy. 16. History of malignancy ≤5 years before enrollment, except for adequately treated basal cell or squamous cell skin cancer or in situ cervical cancer