Pfizer 25-Valent Pneumococcal Vaccine Boosts Serotype 3 Response in Infants

A four-dose series of the investigational 25-valent pneumococcal conjugate vaccine PF-07872412 (25vPnC; Pfizer) elicited immune responses against all 25 included serotypes in healthy infants, according to a new announcement from Pfizer.¹ Detailed phase 2 results were presented in an oral session at IDWeek 2026, held October 21-24 in Washington, DC.¹,² Safety and tolerability were comparable to Prevnar 20 (pneumococcal 20-valent conjugate vaccine; Pfizer).¹

The candidate adds serotypes 15A, 23A, 23B, 24F, and 35B to the 20 covered by Prevnar 20 and uses next-generation conjugate technology aimed at serotype 3.¹ Serotype 3 causes 10.6% of invasive pneumococcal disease (IPD) in US children younger than five years.¹ The five new serotypes are estimated to account for roughly 30% of additional pediatric IPD cases beyond Prevnar 20 coverage.¹

The randomized phase 2 trial (NCT06524414), which began enrollment in July 2024, compared 25vPnC with Prevnar 20 in healthy infants vaccinated at 2, 4, 6, and 12 to 15 months of age.³ Investigators assessed reactogenicity within seven days of each dose, adverse events, and immunogenicity one month after doses 3 and 4.³

Serotype 3 IgG geometric mean concentrations (GMCs) were 8.8-fold higher with 25vPnC one month after dose 3 (4.22 vs 0.48).¹,³ One month after dose 4, serotype 3 GMCs were approximately 15-fold higher with 25vPnC (13.85 vs 0.92).¹,³

Opsonophagocytic Activity and Safety of 25vPnC vs PCV20 in Infants

Beyond binding antibody levels, 25vPnC elicited functional opsonophagocytic activity (OPA) responses against all 25 target serotypes one month after dose 4, consistent with a booster effect.¹ OPA measures the capacity of vaccine-induced antibodies to promote phagocytic killing of pneumococci.¹

Rates of local reactions, systemic events, and adverse events were balanced between the 25vPnC and Prevnar 20 arms.¹ Adverse events were consistent with conditions commonly diagnosed in infants, and no vaccine-related serious adverse events were identified.¹ In an earlier preliminary analysis, the most common local reactions were injection-site redness, swelling, and pain.³

Serotype-level GMC ratios, OPA titers, and adverse event percentages were not included in Pfizer’s announcement.¹

“We changed the conjugation chemistry and we changed the carrier, and then we saw these phenomenal responses of 8.8 and 15 after the third and fourth dose respectively,” said Alejandra Gurtman, MD, senior vice president, Vaccine Research and Development, Pfizer. She noted serotype 3 has persisted at roughly 10% to 11% of US pediatric IPD despite declines in other vaccine serotypes, and said the company hopes the higher antibody levels will translate into clinical protection.

Gurtman said Pfizer selected the new serotypes by tracking emerging replacement serotypes in global surveillance data, citing 23B and 35B as important causes of IPD in children younger than five years. “Pediatricians will be very happy to have a vaccine that continues to provide protection against a serious disease,” she said.

Three phase 3 studies of 25vPnC began in 2026, randomizing up to 6,830 participants to 25vPnC or Prevnar 20, and enrollment is complete in a separate lot-consistency study.¹ If successful, Pfizer estimates the candidate could cover serotypes responsible for approximately 84% of IPD in US children younger than five years.¹ Pfizer also plans to move a 35-valent adult candidate into clinical development by the end of 2026, pending regulatory alignment.¹

REFERENCES

1. Pfizer Inc. Pfizer’s 25-valent pediatric pneumococcal conjugate vaccine candidate shows robust phase 2 data at IDWeek 2026. News release. October 24, 2026. Accessed October 24, 2026. https://www.businesswire.com/news/home/20261024078788/en/

2. Smith M, Flint L, Trammel J, et al. Immunogenicity of a 25-valent pneumococcal coverage vaccine compared with PCV20 in healthy infants: evaluating a candidate to improve pneumococcal coverage in a phase 2 trial. Presented at: IDWeek 2026; October 21-24, 2026; Washington, DC.

3. Pfizer Inc. Pfizer advances pivotal pediatric pneumococcal vaccine program following strong positive phase 2 results. News release. May 20, 2026. Accessed October 24, 2026. https://www.pfizer.com/news/press-release/press-release-detail/pfizer-advances-pivotal-pediatric-pneumococcal-vaccine