A Phase 2 Study to Evaluate the Safety, Tolerability, and Immune Response of AFX3772 Vaccine in Healthy Infants

Inclusion Criteria:

1. Participant has an LAR who understands study procedures, alternative treatments andrisks involved in the study and who voluntarily agrees to participate by givinginformed consent.

2. Participant is a full-term (≥37 weeks gestation at birth) infant approximately 2months of age (42 to 90 days, inclusive) at time of obtaining the informed consent.

3. Participant is healthy based on the clinical judgment of the investigator followingreview of medical history and physical examination.

Exclusion Criteria:

1. Participant had prior administration of any pneumococcal vaccine.

2. Participant has a known or suspected hypersensitivity to AFX3772, PCV13, PCV20 orany components of the formulations used.

3. Participant has a known or suspected immunodeficiency (including autoimmune disease)or other conditions associated with immunosuppression that may requireimmunosuppressive drugs including but not limited to HIV infection, malignancy ororgan or bone marrow transplant. a. In addition, the participant's biological mother has known HIV infection or knownto be hepatitis B surface antigen positive.

4. Participant has any clinically significant allergic condition or history (includinganaphylactic reactions, but excluding untreated, asymptomatic, seasonal allergies)prior to the first vaccination for primary immunization series.

5. Participant has functional asplenia.

6. Participant has medical history of epilepsy or febrile convulsion.

7. Participant weighed <2000 g at birth.

8. Participant has known neurological or cognitive behavioral disorders includingclinically significant developmental disorder and related disorders.

9. Participant has any active malignancy or history of malignancy.

10. Participant has a history of microbiologically proven invasive disease caused by S.pneumoniae.

11. Participant has cochlear implant or cerebrospinal fluid leakage.

12. Has received other licensed non-live vaccines within the 14 days or a licensed livevirus vaccine within the 30 days prior to receipt of study vaccine.

13. Participant has received immunoglobulins. Note: This does not include monoclonalantibodies, such as palivizumab or nirsevimab.

14. Participant has a bleeding diathesis or condition associated with prolonged bleedingthat would contraindicate intramuscular injection.

15. Participant has received systemic corticosteroids (equivalent of >2 mg/kg totaldaily dose of prednisone or >20 mg/day for persons weighing >10 kg) for a period ofmore than 14 days and has not completed the treatment for at least 30 days beforestudy vaccine. Topical, ophthalmic, and inhaled steroids are allowed.

16. Participant has febrile illness (rectal temperature ≥38.1°C [100.5°F] or axillarytemperature ≥37.8°C [100.0°F]) in Part 1 and Part 2 prior to the effective date ofprotocol amendment 4. After protocol amendment 4 becomes effective, febrile illnessis defined as rectal temperature ≥38.0°C [100.4°F] or axillary temperature ≥37.5°C (99.1°F) in Part 2. Upon resolution of symptoms, participant can be rescreened.

17. Participant has received any investigational therapy within 28 days or 5 half-lives,whichever is longer, prior to screening.

18. Participant has any condition which, in the investigator's opinion, makes theparticipant unsuitable for study participation.

19. LARs that are study personnel or their immediate dependents, family, or householdmembers.