Pfizer says its 25-valent paediatric vaccine produced substantially higher antibody responses against serotype 3 than Prevnar 20 in infants, as the candidate moves into phase 3 testing.
Pfizer said new data from its 25-valent pneumococcal conjugate vaccine candidate in infants showed a stronger response against serotype 3 than its current 20-valent shot, a strain that has remained stubbornly difficult to control even as other vaccine-covered types have fallen. The findings, presented by the company at IDWeek 2026 and outlined in its announcement, add to the case for expanding paediatric protection against an infection that can cause pneumonia, bloodstream infection and meningitis.
According to Pfizer, the phase 2 study found that 25vPnC produced robust immune responses to all 25 targeted serotypes and generated particularly marked gains against serotype 3. The company said geometric mean concentrations were 8.8 times higher after the third dose and about 15 times higher after the fourth dose compared with Prevnar 20. Alejandra Gurtman, senior vice president for vaccine research and development at Pfizer, said the company changed both the conjugation chemistry and the carrier used in the vaccine and then saw “phenomenal responses” to serotype 3.
Pfizer also said the candidate triggered functional opsonophagocytic activity, or OPA, across all 25 serotypes one month after the fourth dose. OPA measures how well antibodies can mark bacteria for destruction by immune cells, so it is often used as a functional check on whether a vaccine is doing more than raising antibody levels alone. The company said the safety profile looked balanced between the 25-valent vaccine and Prevnar 20, with no vaccine-related serious adverse events identified.
The focus on serotype 3 reflects a broader problem for pneumococcal vaccine developers. Gurtman said the strain still accounts for roughly 10% to 11% of invasive pneumococcal disease in US children, even after declines in many other vaccine serotypes. Pfizer has also said it selected additional serotypes by monitoring global replacement patterns, pointing to 23B and 35B as important causes of serious disease in children under five. If the expanded formulation clears later-stage testing, the company estimates it could cover serotypes responsible for about 84% of invasive pneumococcal disease in US children younger than five.
Pfizer has already started three phase 3 studies of 25vPnC, enrolling up to 6,830 participants, and says a separate lot-consistency study has finished recruitment. The company is also moving ahead with a 35-valent adult candidate, subject to regulatory alignment, as it tries to build a broader pneumococcal portfolio around both paediatric and adult use.
Disclaimer: This content is for informational purposes only and is not intended to be a substitute for professional medical judgment, advice, diagnosis, or treatment.
