Safety, Tolerability, and Immunogenicity of VAX-31 in Adults ≥50 Years With Prior Pneumococcal Vaccination

Inclusion Criteria: * Male or female ≥50 years of age (inclusive) at the time of randomization into the study. * Previous receipt of a licensed pneumococcal vaccine or combination of licensed vaccines, with most recent vaccination ≥1 year prior to randomization; the exception is PCV21, which may have been received ≥6 months prior to randomization (confirmed). * Able and willing to complete the informed consent process. * Available for clinical follow-up through the last study visit. * In good general health or with stable underlying chronic condition(s), as determined by medical history, oral temperature, physical examination, and clinical judgment of the Investigator (ongoing chronic conditions must be documented as stable per Investigator). * Willing to have blood samples collected and used for research purposes. * Able to provide proof of identity to the satisfaction of the site personnel completing the enrollment process. * Female participants 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. Male subjects with partners of childbearing potential must agree to practice an acceptable contraception method. * Able to access and use a device connected to Wi-Fi or cellular network for completion of an electronic diary (eDiary). Exclusion Criteria: * Previous invasive pneumococcal disease (IPD) or pneumococcal pneumonia (either confirmed or self-reported) at any age. * Previous receipt of an investigational pneumococcal vaccine at any age. * Receipt of any investigational product within 30 days prior to Day 1, currently participating in another interventional investigational study, or having plans to receive another investigational product(s) while on study. * Receipt of any live vaccine within 30 days prior to Day 1, or receipt of any non-live (including inactivated) vaccine within 14 days prior to Day 1. * Body temperature \>38.0°C (\>100.4°F) or acute illness within 3 days prior to study vaccination (subject may be rescreened). * Current diagnosis of human immunodeficiency virus, Hepatitis B, or Hepatitis C. * History of severe allergic reaction with generalized urticaria, angioedema, or anaphylaxis to any previous vaccination. * Individual who is pregnant, breastfeeding, or planning to become pregnant during study participation. * Has a known or suspected immunocompromising condition, including, but not limited to, leukemia, lymphoma, chronic renal failure, or congenital or acquired immunodeficiency. * 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. * Receipt of blood or blood product (including polyclonal intravenous immunoglobulin) within 60 days prior to enrollment into the study. * Is currently receiving immunosuppressive or immune-modifying therapy, including systemic corticosteroids (this includes ≥3 months of prednisone equivalent from 5 to ≤10 mg/day and ≥2 weeks of prednisone equivalent \>10 mg/day). * Received any part of a ≥14-day course of systemic corticosteroids (prednisone equivalent \>10 mg/day) within 14 days of study vaccination • History of malignancy ≤5 years before enrollment, except for adequately treated basal cell or squamous cell skin cancer or in situ cervical cancer. * 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. * Employee of, or first-degree relative of, any person employed by the Sponsor, the contract research organization (CRO), the Investigator, site personnel, or site