The US Food and Drug Administration has recommended COVID-19 vaccine manufacturers change the design of their booster shots to combat coronavirus sub-variants.

If authorised, the changes would mark the first major retooling of COVID vaccines, but also could slow their rollout.

The FDA will not require new studies testing the shots designed to battle the currently dominating Omicron BA.4/BA.5 strains, a top agency official told Reuters.

"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 Centre, adding that redesigned vaccines could provide better protection than 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, targeting both the original virus as well as the Omicron subvariants.

BA.4 and BA.5 are estimated to account for more than 50 per cent of US infections, according the US Centres for Disease Control and Prevention, and have also become dominant elsewhere.

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

Scientists have suggested redesigned boosters could spur a broader range of immune responses that might also protect against future variants that may emerge.

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

Advisers to the World Health Organisation preferred BA.1-based boosters, arguing that the variant is more distinct and could generate a broader response than the more recently circulating sub-variants.

The FDA has 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 authorisation of the retooled shots.