Vaxcyte said on Monday its experimental pneumococcal vaccine VAX-31 met its main goals in a pivotal late-stage trial in adults 50 and older, sending shares up sharply. The stock rose as much as 91 percent before the bell and opened at a 20-month high near $89.

The OPUS-1 Phase 3 trial tested the vaccine against Pfizer's Prevnar 20 and Merck's Capvaxive. VAX-31 produced immune responses at least comparable to both rivals across the 28 bacterial strains, or serotypes, it shares with one or both vaccines.

It also produced stronger immune responses against the three strains unique to VAX-31 and against strain 20B. Leerink analyst David Risinger said he expects physicians and patients to prefer VAX-31 for its broader protection. The Prevnar family and Capvaxive generated $7.25 billion in sales last year.

Pneumococcal disease is caused by bacteria that can lead to pneumonia, meningitis and bloodstream infections, with older adults particularly at risk. The trial showed a consistent safety profile across age groups, with no serious adverse events considered related to the vaccine.

Vaxcyte expects results from two more late-stage trials in the first half of 2027 and plans to file for US marketing approval in the first half of 2028.