Pfizer’s 25‑valent infant pneumococcal vaccine showed immune responses to all 25 serotypes in Phase 2, company reports

Pfizer said Saturday that its investigational 25-valent pneumococcal conjugate vaccine for infants generated immune responses against all 25 targeted serotypes in a Phase 2 study, with safety and tolerability comparable to Prevnar 20. The company reported the results in a press release dated Oct. 24 and said the data were presented in an oral session at IDWeek 2026 in Washington, D.C.

The update matters because Pfizer is trying to expand protection beyond today’s 20-valent pediatric standard against strains of Streptococcus pneumoniae that still cause serious illness in children. Those infections can lead to bacteremia, meningitis and pneumonia. The candidate, known as PF-07872412 or 25vPnC, adds five serotypes not covered by Prevnar 20 and is also designed to improve responses against serotype 3, a strain that has remained a notable residual cause of invasive disease in some surveillance data despite inclusion in earlier vaccines.

According to Pfizer, the Phase 2 trial showed “robust immune responses against all 25 serotypes” one month after the third dose and again one month after the fourth dose. The company also said the vaccine produced functional opsonophagocytic activity, or OPA, responses for all 25 serotypes one month after dose 4. OPA is a lab measure of whether vaccine-induced antibodies can help immune cells kill pneumococcal bacteria and is commonly used in pneumococcal vaccine development.

Pfizer highlighted one result in particular for serotype 3. The company said immune responses for that strain were 8.8 times higher after dose 3 and about 15 times higher after dose 4 than with Prevnar 20. Pfizer gave the underlying values as 4.22 versus 0.48 after dose 3 and 13.85 versus 0.92 after dose 4.

On safety, Pfizer said rates of local reactions, systemic events and adverse events were balanced between the 25vPnC and Prevnar 20 groups, and that no related serious adverse events were identified. Among events reported in more than 10% of infants on a four-dose schedule were irritability, injection-site pain, drowsiness, decreased appetite and fever, according to the company.

The five added serotypes are 15A, 23A, 23B, 24F and 35B. Pfizer said those strains could account for about 30% additional pediatric invasive pneumococcal disease cases in the United States compared with Prevnar 20, and that the 25-valent candidate could cover serotypes responsible for about 84% of invasive pneumococcal disease cases in U.S. children younger than 5. Pfizer said those are company estimates based on Centers for Disease Control and Prevention Active Bacterial Core surveillance data.

“These encouraging data build on more than 25 years of progress in protecting children from serious pneumococcal disease with PREVNAR vaccines,” Annaliesa Anderson, Pfizer’s senior vice president and chief vaccines officer, said in the company’s statement.

The Phase 2 trial began enrollment in July 2024, Pfizer said. The company added that the vaccine is already being tested in three Phase 3 pediatric trials that began earlier in 2026, with up to 6,830 participants randomized across the program to receive either 25vPnC or Prevnar 20. Pfizer also said it has completed enrollment in a Phase 3 lot-consistency study.

The findings should be viewed as company-reported conference data rather than proof of clinical effectiveness. Based on the materials reviewed, the full results had not been described in a peer-reviewed journal article as of Oct. 24.