The goal of this clinical trial is to evaluate whether the wSp vaccine can prevent nasopharyngeal colonization by pneumococcal bacteria in healthy young children who have received routine PCV15 or PCV20 vaccination.

The main questions it aims to answer are:

Does wSp vaccine reduce pneumococcal colonization in the nose? Does wSp vaccine safely stimulate the immune system to produce antibodies and protective immune responses?

Researchers will compare children receiving two injections of wSp vaccine to those receiving saline placebo to see if wSp vaccine reduces colonization and boosts immunity.

Participants will:

Receive two injections of either wSp or saline at approximately 7 and 9 months of age.

Have blood samples taken at 6 and/or 12 months to measure immune responses. Have nasal swabs collected at several time points and during infections from 6-24 months.

Be monitored for ear infections, respiratory illnesses, and other pneumococcal-related infections.

Inclusion Criteria:

1. Male or female who is 6 months (- 30 to +75 days) of age at time of enrollment
2. Healthy subject as established by medical history and clinical examination before entering into the study.
3. Received 3 doses of PCV15 or PCV20.
4. Written informed consent obtained from the subject's parent/legal guardian.
5. Parent/legal guardian able and willing to bring subject to all study visits.

Exclusion Criteria:

1. Has a history of invasive pneumococcal disease (positive blood culture and/or positive cerebrospinal fluid culture).

2. Known or suspected impairment of immunological function, based on medical history and physical examination.

3. Has a history of congenital or acquired immunodeficiency.

4. Chronic administration (defined > 14 consecutive days) of immunosuppressants or other immune-modifying drugs.

5. Prior receipt since birth of immunoglobulins and/or any blood products or planned administration during the study period.

6. Expected to receive systemic corticosteroids within 14 days prior to any dose of study vaccine.

7. External auditory canal atresia/stenosis.

8. Has known or history of functional or anatomic asplenia.

9. Has a bleeding disorder in which intramuscular vaccination would be contraindicated.

10. Participation in another investigational or interventional trial within the 28-day period before enrollment and during the conduct of the study. Participation in observational studies is permitted.

11. Direct descendant (child or grandchild) of study site personnel.

    Temporary exclusion criteria

    For day of vaccination:

12. Fever (transcutaneous temperature ≥38.0°C) or acute illness, at investigator's discretion

13. Has received systemic corticosteroids (equivalent of prednisone > 0.5 mg/kg/day or equivalent) and not completed this course of treatment at least 30 days prior to the study vaccination. Inhaled, topical, and ophthalmic steroids are allowed.

Only the person administering the investigational product will be unmasked. This person does not perform any other participant-related tasks in the clinical trial.