Pfizer’s 25-valent pneumococcal vaccine shows robust immune responses in Phase II infant study

Pfizer has reported positive Phase II results for its investigational 25-valent pneumococcal conjugate vaccine candidate, PF-07872412, in healthy infants.

The findings were presented at IDWeek 2026 and showed that a four-dose series of the vaccine generated robust immune responses against all 25 targeted pneumococcal serotypes.

The investigational vaccine, also known as 25vPnC, was compared with Pfizer’s approved 20-valent pneumococcal conjugate vaccine, PREVNAR 20.

Pfizer said immune responses against the 20 serotypes shared by both vaccines were comparable, while 25vPnC also generated responses against five additional serotypes.

These additional serotypes are 15A, 23A, 23B, 24F and 35B.

Responses were observed one month after the third dose and again one month after the fourth dose.

The company said the vaccine could potentially provide protection against serotypes responsible for approximately 84% of invasive pneumococcal disease cases among children under five in the U.S.

The Phase II study also assessed the vaccine’s response against serotype 3, which has historically been challenging to target effectively.

One month after the third dose, geometric mean antibody concentrations against serotype 3 were 8.8 times higher with 25vPnC than with PREVNAR 20.

Following the fourth dose, concentrations were approximately 15 times higher with the investigational vaccine.

Pfizer said the candidate uses next-generation technology designed specifically to generate a stronger immune response against serotype 3.

The serotype accounts for an estimated 10.6% of invasive pneumococcal disease cases among U.S. children younger than five.

Functional immune responses were also observed across all 25 targeted serotypes using an opsonophagocytic activity assay one month after the fourth dose.

These findings indicate that the antibodies generated by vaccination were capable of supporting the immune system’s ability to kill pneumococcal bacteria.

Annaliesa Anderson, PhD, Senior Vice President and Chief Vaccines Officer at Pfizer, said the candidate builds on more than 25 years of development of the company’s PREVNAR vaccines.

She said expanding serotype coverage while strengthening the response against serotype 3 could potentially provide broader protection against invasive pneumococcal disease in children.

The safety and tolerability profile of 25vPnC was comparable with PREVNAR 20 in the Phase II study.

Rates of local reactions, systemic events and adverse events were balanced between the two vaccine groups.

Reported adverse events were consistent with conditions commonly seen in infants, and no treatment-related serious adverse events were identified.

The Phase II findings support Pfizer’s ongoing Phase III development programme evaluating the 25-valent vaccine candidate in the paediatric population.

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