This study will evaluate the safety and immune response of a new formulation of pneumococcal vaccine, PnMAPS30plus, in healthy adults aged 50 to 64 years. Participants will receive a single dose of either the investigational vaccine or an approved pneumococcal vaccine (PCV20) and will be monitored for approximately six months. The study aims to determine if PnMAPS30plus is safe and well-tolerated and whether it helps the body produce antibodies that protect against pneumococcal disease.

Inclusion Criteria:

1. Participants who, in the opinion of the investigator, can comply with the requirements (e.g., completion of the electronic diary [eDiary], return for follow-up visits).
2. Written or witnessed/thumb printed informed consent obtained before any trial-specific procedure.
3. Healthy or medically stable adults confirmed by medical history and clinical examination at screening. Participants with stable medical conditions can be enrolled at the discretion of the investigator.
4. Male and female participants aged 50 and 64 years of age (YOA) at the time of Informed consent form (ICF) signature.
5. Female participants of childbearing potential (POCBP) may enrol if they practice adequate contraception and have a negative pregnancy test prior to study intervention administration.

Exclusion Criteria:

1. History of microbiologically proven invasive pneumococcal disease (IPD) caused by S. pneumoniae within 3 years before study intervention administration.
2. History of any reaction or hypersensitivity likely to be exacerbated by any component of the study intervention(s).
3. Any confirmed or suspected immunosuppressive or immunodeficient condition.
4. Hypersensitivity to latex.
5. Clinical conditions representing a contraindication to intramuscular vaccination, and blood collection.
6. Documented HIV-positive status.
7. Acute or unstable chronic clinically significant pulmonary, cardiovascular, hepatic or renal functional abnormality, or metabolic conditions as determined by the investigator.
8. Recurrent history of uncontrolled neurological or any neuroinflammatory disorders.
9. Any behavioural or cognitive impairment or psychiatric disease that, in the opinion of the investigator, may interfere with the participant's ability to participate in the trial.
10. Any other clinical condition that, might pose additional risk to the participant.
11. Use of any investigational or non-registered product other than the study interventions within 30 days before the study intervention administration.
12. History of previous vaccination with any pneumococcal vaccine.
13. Administration of immunoglobulins or other blood products or plasma derivatives within 90 days before or planned within 30 days after study intervention.
14. Chronic administrations of immune-modifying drugs (defined as more than 14 consecutive days in total) and/or planned use of long-acting immune-modifying treatments at any time up to the end of the trial.
15. Pregnant or lactating female participant.
16. History of and/or current chronic alcohol consumption and/or drug abuse, based on investigator judgment.

Study Sponsor will assess requests from qualified researchers for anonymized individual patient-level data and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/About\_GSK\_Patient\_Level\_Data\_Sharing\_Final\_13July2023.pdf

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or asset(s) with development terminated across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months, but an extension may be granted, when justified, for up to 6 months.

This trial will be conducted in an observer-blind manner.