A combination Covid-flu vaccination developed by Pfizer Inc. and BioNTech SE missed on one of its goals in a final-stage trial, a disappointment for the companies trying to make further use of the technology that succeeded in the pandemic.

The vaccine failed to show it was at least as effective as a standard shot in generating an immune response against the influenza B strain, the companies said in a statement Friday. The shot performed better with influenza A and Covid. Pfizer said the two companies are looking into tweaks to the vaccine that might boost its effectiveness against influenza B.

Pfizer shares fell 0.3% before US markets opened, while BioNTech’s depositary receipts lost 4%.

What Bloomberg Intelligence Says

The Phase 3 trial failure of Pfizer-BioNTech’s combination flu/Covid vaccine may put the partners at a disadvantage vs. Moderna, which succeeded in Phase 3 in June. We see the combination shots as an important marketing and commercialization tool for the three companies to boost penetration of the respiratory-disease vaccine, helping them rival the likes of GSK and Sanofi. A reworked combination may set Pfizer-BioNTech back by at least two years, and would need a new Phase 3 trial.

— BI analyst Sam Fazeli

The results complicate Pfizer’s efforts to offset the drop in sales of its BioNTech-partnered Covid vaccine, which fell more than 70% last year. Moderna Inc., maker of a rival Covid vaccine, reported positive late-stage results with a Covid-flu combination of its own in June.

Pfizer’s study, which enrolled more than 8,000 adults, compared the combination shot against an approved flu vaccine. The company also disclosed results from a smaller, earlier-stage trial in which a second-generation combination vaccine led to a sufficient immune response against the A and B strains of influenza.