Pfizer Inc. and BioNTech SE vaccines modified to target the Omicron variant produced a significantly larger immune response than the companies’ currently available vaccine in a study, they said.

A modified booster shot targeting Omicron specifically increased neutralizing antibody levels 13.5 to 19.6 times higher than the current shot in study volunteers a month after administration, depending on the dose, the companies said Saturday.

A booster targeting both Omicron and the original virus increased neutralizing antibody levels 9.1 to 10.9 times, depending on the dose, the companies said.

Both of the tweaked vaccine candidates were well tolerated by subjects in the study, the companies said.

The results, coming after Moderna Inc. also found its Omicron-targeting booster produced a stronger immune response, suggest possible benefit to modifying the shots to improve protection against an evolving virus.

Federal health authorities are trying to decide whether to stick with the current shots for a fall vaccination campaign or use a tweaked version. Studies have found that the current vaccines don’t work as well against Omicron as they did against earlier strains.

“Based on these data, we believe we have two very strong Omicron-modified candidates that elicit a substantially higher immune response against Omicron than we’ve seen to date,” Pfizer Chief Executive Albert Bourla said.

The study didn’t measure whether and how well the shots reduced the risk of Covid-19. Pfizer and BioNTech announced the results by news release. The findings haven’t been published in a peer-reviewed medical journal.

Omicron is the most recent strain of the virus to come to predominate in the U.S. and many other countries.

An expert panel will meet soon to advise the FDA on whether and how to modify Covid-19 vaccines; a Covid-19 testing and vaccination site in New York in April.

Photo: Spencer Platt/Getty Images

During a wave of Omicron cases, hospitalizations rose to record highs, including among children, and some treatments and vaccines were found to be less effective than they had been against earlier strains.

Current vaccines did reduce the risk of hospitalization and death, however, according to their makers.

While the winter’s Omicron wave has receded, health authorities express worry that a new subvariant or variant could emerge even better able to evade existing vaccines.

To strengthen their inoculations, vaccine makers have been working on tweaking their shots, including developing shots that target more than one strain of the virus.

Pfizer and BioNTech “remain vigilant and are prepared to rapidly adapt our Omicron-based vaccine to emerging sublineages if epidemiological and laboratory data suggest,” BioNTech Chief Executive Ugur Sahin said.

The companies tested their two modified vaccines in a late-stage trial with 1,234 subjects who were 56 years and older.

Pfizer and BioNTech tested two doses—30 micrograms and 60 micrograms—of each modified shot. Adults get a 30 microgram dose of the companies’ current shot.

A month after giving the shots, the companies compared the levels of neutralizing antibodies produced by the doses to the levels generated by the current Pfizer-BioNTech vaccine.

Neutralizing antibodies are among the immune system soldiers protecting against a virus.

Early lab testing showed the modified vaccines were also effective against the Omicron subvariants BA.4 and BA.5, but lost some of their power, generating about three times lower antibody levels against those subvariants than against the original Omicron subvariant, according to the companies. Pfizer and BioNTech said they would collect more data related to the sub variants in the coming weeks.

A panel of vaccine experts is scheduled to meet Tuesday to advise the Food and Drug Administration on whether and how to modify the Covid-19 vaccines, and representatives from Moderna and Pfizer are expected to discuss the data from testing on their modified shots.

Write to Jared S. Hopkins at jared.hopkins@wsj.com and Jonathan D. Rockoff at Jonathan.Rockoff@wsj.com