This is a Phase 2 clinical study to support the use of AFX3772 in healthy infants for the prevention of pneumococcal disease. The purpose of this study is to determine the safety, tolerability, and immunogenicity of 3 different formulations of AFX3772 compared with Prevnar 13 (PCV13) and Prevnar 20 (PCV).

Part 1 is the dose escalation, lead-in portion of the study in which infants at each dose level will be randomized 3:1 in sequential cohorts of increasing doses of AFX3772 or PCV13.

In Part 2, infants will be randomized to receive either one of two dose levels of AFX3772 or PCV20.

Inclusion Criteria:

• Is a full-term infant approximately 2 months of age at time of obtaining the informed consent.

Exclusion Criteria:

• Had prior administration of any pneumococcal vaccine.
• Has a known or suspected hypersensitivity to AFX3772, PCV13, PCV20 or any components of the formulations used.
• Has a known or suspected immunodeficiency or other conditions associated with immunosuppression that may require immunosuppressive drugs. In addition, the participant's biological mother has known HIV infection or known to be hepatitis B surface antigen positive.
• Has any clinically significant allergic condition or history prior to the first vaccination for primary immunization series.
• Has a history of microbiologically proven invasive disease caused by S. pneumoniae.
• Has received immunoglobulins.
• Has a bleeding diathesis or condition associated with prolonged bleeding that would contraindicate intramuscular injection.
• Has received systemic corticosteroids for a period of more than 14 days and has not completed the treatment for at least 30 days before study vaccine.

GSK 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 terminated asset(s) 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.