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The U.S. Food and Drug Administration on Thursday recommended COVID-19 vaccine manufacturers change the design of their booster shots beginning this fall to include components tailored to combat the currently dominant Omicron BA.4 and BA.5 subvariants of the coronavirus.

If authorized, the changes would mark the first major retooling of the COVID vaccines, but also could slow their rollout as the FDA has recommended a design somewhat different from what the companies had already tested and started producing.

The FDA also has suggested it could authorize the new shots before studies testing them in humans are completed. Annual changes to flu vaccines do not require new trials.

“In a sense, we are chasing the virus, just as we do with influenza, and how close we can get to the variants that are prominent at the time, we’ll have to wait and see,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, adding that the redesigned vaccines could provide better protection than the current ones.

The FDA said companies would not need to change the vaccine for the primary vaccination series, saying the coming year will be “a transitional period when this modified booster vaccine may be introduced.”

The new booster shots would be bivalent vaccines, meaning doses would target both the original virus as well as the Omicron subvariants.

The decision follows a recommendation by the agency’s outside advisers to change the design of the shots this fall in order to combat more prevalent versions of the coronavirus.

BA.4 and BA.5 are now estimated to account for more than 50% of U.S. infections, according the U.S. Centers for Disease Control and Prevention, and have also become dominant elsewhere.

The FDA said in a statement on Thursday that it hoped the modified vaccines could be used in early to mid-fall.

Scientists have suggested that introducing redesigned boosters could spur broader range of immune responses that might also protect against future variants, even if they are not the ones currently circulating.

“Predicting the future with COVID-19 in particular is hazardous, because COVID has faked us out on a number of occasions,” Schaffner said.

Pfizer Inc with partner BioNTech SE and Moderna Inc have been testing versions of their vaccines modified to combat the BA.1 Omicron variant that caused the massive surge in cases last winter.

Although they have said those vaccines generated a good immune response against BA.1 and the more recently circulating variants, they did see a lower response against BA.4 and BA.5.

The companies had already been manufacturing their BA.1 vaccines, and said on Tuesday switching to a BA.4/BA.5 design could delay their introduction.

Pfizer/BioNTech, which on Wednesday announced a $3.2 billion contract to supply more COVID vaccine doses to the United States, said they would have a substantial amount of BA.4/BA.5 vaccine ready for distribution by the first week of October.

Moderna said it would be late October or early November before it would have the newly modified vaccine ready.

The exact authorization process for the new shot is not yet clear.

The FDA directed manufacturers to launch clinical trials to study the BA.4/5 vaccines, but said they would evaluate data on their previous BA.1-based vaccines to support authorization of the modified shots.

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This content appears as provided to The Globe by the originating wire service. It has not been edited by Globe staff.