Pfizer’s 25-valent infant pneumococcal vaccine clears its phase 2 test
Full phase 2 results presented at IDWeek show Pfizer’s next infant pneumococcal vaccine outperforming Prevnar 20 on serotype 3. The commercial stakes in the paediatric market are large, and phase 3 is already under way.
In brief
In phase 2, Pfizer’s 25-valent infant pneumococcal vaccine induced responses to all 25 serotypes and far stronger serotype 3 responses than Prevnar 20.
- Phase 2 data for Pfizer’s 25-valent vaccine were presented at IDWeek on 24 October 2026.
- Serotype 3 antibody concentrations were 8.8-fold higher than with Prevnar 20 after the third dose and about 15-fold higher after the fourth.
- Three paediatric phase 3 studies with up to 6,830 participants began earlier in 2026.
Pfizer presented full phase 2 results for its 25-valent pneumococcal conjugate vaccine candidate, PF-07872412, at IDWeek in Washington on 24 October 2026. The company had first reported top-line findings in May. The candidate is designed to succeed Prevnar 20, Pfizer’s current infant vaccine, adding five serotypes: 15A, 23A, 23B, 24F and 35B.
The study, registered as NCT06524414, enrolled 605 healthy infants at US sites, according to ClinicalTrials.gov, and compared the candidate with Prevnar 20 on a four-dose schedule given at two, four, six and 12 to 15 months.
Serotype 3 is the headline
Pfizer reported robust immune responses against all 25 serotypes one month after the third and fourth doses, and functional antibody responses against all 25 after the fourth dose. The most notable result was for serotype 3, which continues to cause disease in young children. Geometric mean antibody concentrations were 8.8 times those seen with Prevnar 20 one month after dose three, and about 15 times higher one month after dose four.
Citing US surveillance data, Pfizer says serotype 3 accounts for 10.6% of invasive pneumococcal disease in children under five. The candidate uses what the company describes as next-generation technology intended to produce a stronger serotype 3 response.
Each added serotype is a manufacturing and immunological trade-off, so a broader vaccine must show it has not diluted the response to the serotypes it already covers.
Safety and coverage claims
Local reactions, systemic events and adverse events were balanced between the two groups, and no vaccine-related serious adverse events were identified. Pfizer estimates that the five new serotypes would cover about 30% more cases of paediatric invasive pneumococcal disease in the US than Prevnar 20, and that the vaccine could protect against serotypes responsible for about 84% of cases in children under five. Both figures depend on phase 3 success.
The regulatory route
Pneumococcal conjugate vaccines are generally licensed on immune responses rather than on disease outcome trials, which makes non-inferiority against the existing standard the central test. Pfizer says three paediatric phase 3 studies began earlier in 2026, randomising up to 6,830 participants to the new vaccine or Prevnar 20, and that a study of consistency across manufacturing lots has completed enrolment.
Why it matters
Infant pneumococcal vaccination is among the largest franchises in paediatric medicine. A product that combines broader valency with a stronger serotype 3 response would widen protection on two fronts at once. Phase 3 will show whether those gains hold across the full panel of serotypes in larger, more varied populations.
