Vaxcyte Rises on Phase 1/2 Results for 31-Valent Pneumococcal Vaccine

Vaxcyte shares, identified as $PCVX, surged by more than 35% on Tuesday morning following encouraging results from a Phase 1/2 clinical trial for its 31-valent pneumococcal conjugate vaccine, VAX-31. This vaccine is currently being tested in adults over 50 years of age.

The clinical trial explored three different dose levels of VAX-31 and compared them against Prevnar 20, a well-established pneumococcal vaccine, in an observer-blind study. Vaxcyte reported that at both the middle and high doses, VAX-31 exhibited non-inferior immune responses across all 20 serotypes shared with Prevnar 20. These immune responses were measured using opsonophagocytic activity (OPA) levels, a standard metric in vaccine efficacy studies.

Moreover, at the high dose, VAX-31 demonstrated a superior average OPA immune response for 18 out of the 20 common serotypes, showing a “geometric mean ratio” (GMR) of at least 1.0. Notably, seven of these serotype responses were statistically significantly higher. The middle dose of VAX-31 also performed well, achieving a GMR of at least 1.0 for 13 out of the 20 common serotypes, with five of these serotypes showing significantly higher responses.

Across all three dose levels, VAX-31 met the trial's criteria for superiority for the 11 serotypes not shared with Prevnar 20, indicating broad efficacy.

Last month, Mizuho analyst Salim Syed commented that for the trial to be considered a success on an individual serotype basis, an OPA GMR of at least 0.6 would be sufficient. According to Syed, achieving an OPA GMR higher than 0.6 is likely predictive of a successful outcome in Phase 3 trials. He also noted that a successful aggregate result would be defined by having six or fewer serotypes fail to meet the 0.6 threshold, though he acknowledged some flexibility given the complexities of the field.

Looking ahead, Vaxcyte plans to initiate a Phase 3 non-inferiority study by mid-2025, with topline data expected to be available in 2026. The company is set to finalize the optimal dose for this future study before its commencement.

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