Pfizer Reports Phase 2 Results for 25-Valent Pediatric Vaccine

NEW YORK — October 24, 2026

Immune Responses Across All 25 Serotypes

The Phase 2 study, registered as NCT06524414, assessed the safety, tolerability and immunogenicity of 25vPnC compared with PREVNAR 20 in healthy infants. Pfizer reported robust immune responses against all 25 serotypes following the four-dose schedule, with assessments conducted one month after Dose 3 and one month after Dose 4. In addition to the 20 serotypes shared with PREVNAR 20, the candidate includes five additional serotypes: 15A, 23A, 23B, 24F and 35B. These additions are intended to address a broader range of pneumococcal strains responsible for invasive disease in children. A notable finding was the enhanced immune response against serotype 3. The candidate generated GMCs approximately 8.8-fold higher after Dose 3 and 15-fold higher after Dose 4 compared with PREVNAR 20. GMCs measure the average antibody concentration on a geometric scale and are commonly used to assess vaccine immunogenicity. Pfizer also reported functional responses in an opsonophagocytic activity (OPA) assay one month after Dose 4 for all 25 serotypes. OPA assesses whether antibodies can support immune-cell-mediated killing of pneumococcal bacteria, providing information beyond antibody concentration alone. The company described the findings as evidence of booster responses across the vaccine’s targeted serotypes. However, immune responses and laboratory measures are not equivalent to demonstrated prevention of infection or disease. The extent to which these findings translate into clinical protection will need to be established through the ongoing clinical development program. The study results provide a basis for evaluating whether the broader serotype coverage and improved response against serotype 3 can extend the protection offered by existing pediatric pneumococcal vaccines.

Safety Findings Comparable to PREVNAR 20

Pfizer reported that the 25-valent candidate had a safety and tolerability profile comparable to PREVNAR 20 in the Phase 2 study. Rates of local reactions, systemic events and adverse events were balanced between the treatment groups, and reported adverse events were consistent with conditions commonly diagnosed in the infant age group. No related serious adverse events were identified in the study. These findings support continued assessment of the candidate in larger trials, where safety can be evaluated across more participants and with additional follow-up. PREVNAR 20 is an approved pneumococcal conjugate vaccine that helps protect against disease caused by 20 Streptococcus pneumoniae serotypes in eligible populations. Pfizer’s investigational candidate seeks to expand serotype coverage while applying next-generation technology to improve the immune response against serotype 3. Pneumococcal disease can cause serious infections, including bacteremia, meningitis and pneumonia, with young children among the populations at risk. Invasive disease occurs when pneumococcal bacteria enter normally sterile sites in the body. Vaccination is an important prevention strategy, but the degree of protection depends on the vaccine, targeted serotypes and population studied. The Phase 2 safety results are encouraging in the context of pediatric vaccine development, but they should be interpreted within the study’s size and duration. Larger Phase 3 studies will provide further evidence about the candidate’s safety, immunogenicity and consistency of performance. The findings do not change the current approved indications for PREVNAR 20, and 25vPnC is not yet approved for use..

Phase 3 Program Advances Pediatric Vaccine Development

Pfizer reported that three Phase 3 studies of 25vPnC began earlier in 2026, with up to 6,830 participants expected to be randomized to receive either the investigational candidate or PREVNAR 20. The company has also completed enrollment in a Phase 3 study designed to demonstrate consistency across three manufacturing lots of the candidate. These studies are intended to build on the Phase 2 immunogenicity and safety findings and provide a larger evidence base for future regulatory assessment. Pfizer estimates that the five additional serotypes could address approximately 30% more pediatric IPD cases in the United States than the serotypes covered by PREVNAR 20, while the full set of 25 serotypes collectively accounts for approximately 84% of cases among children under five. These estimates describe the serotype distribution of disease, not the percentage of cases that the vaccine has already been proven to prevent. The company’s broader pneumococcal vaccine strategy also includes a planned fifth-generation, 35-valent candidate for adults, which Pfizer intends to bring into clinical development by the end of 2026, pending alignment with regulatory authorities. The pediatric 25-valent candidate represents a potential next step in expanding pneumococcal vaccine coverage. Its eventual clinical role will depend on Phase 3 results, regulatory review and evidence that its immune responses translate into meaningful protection. For now, the Phase 2 findings demonstrate broad immunogenicity across the targeted serotypes, stronger measured responses against serotype 3 and a safety profile that Pfizer described as comparable to PREVNAR 20. The ongoing development program will determine whether these findings support a new option for preventing pneumococcal disease in children.

Source:Pfizer ,Press release