Pfizer Inc. (NYSE:PFE) on Wednesday reported positive Phase 2 data for its investigational 25-valent pneumococcal conjugate vaccine candidate.

The study showed strong immune responses in infants compared with PREVNAR 20, supporting advancement into Phase 3 testing.

Stronger Immune Response Against Serotype 3

According to Pfizer, the vaccine candidate, known as 25vPnC, generated notably stronger immune responses against serotype 3, a major remaining cause of invasive pneumococcal disease and complicated pneumonia in children.

One month after the third dose, geometric mean titers for serotype 3 were 8.8 times higher with 25vPnC than PREVNAR 20.

After the fourth dose, titers were roughly 15 times higher than the comparator vaccine.

Pfizer said the vaccine candidate could potentially cover up to 90% of disease-causing serotypes in children younger than five years old, including about 15% linked to serotype 3.

Read Also: Why Your Pneumonia Shot Is About To Get A Massive Upgrade

Pfizer Launches Pediatric Phase 3 Program

The randomized Phase 2 trial enrolled healthy infants beginning in July 2024. Participants received either 25vPnC or PREVNAR 20 at two, four, six, and 12-15 months of age.

Pfizer said the safety and tolerability profile of 25vPnC was consistent with currently approved pneumococcal vaccines. The most common side effects included redness, swelling, and pain at the injection site.

Based on the Phase 2 data and regulatory discussions, Pfizer initiated a pivotal pediatric Phase 3 program in May 2026 involving up to 2,400 participants.

Adult Vaccine Program Moves Toward 35-Valent Candidate

Pfizer also said it plans to move directly to a fifth-generation adult pneumococcal vaccine candidate covering 35 serotypes as part of its long-term strategy in the adult vaccine market.

The company expects the adult candidate to enter clinical development by the end of 2026, pending regulatory alignment.

PFE Price Action: Pfizer shares were up 0.16% at $25.70 during premarket trading on Wednesday, according to Benzinga Pro.

Over the past month, PFE has declined about 6.8% versus a 3.5% rise in the S&P 500 and is up roughly 3% year-to-date compared to the index’s 7.0% gain.

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