The US Food and Drug Administration on Wednesday authorised Moderna and Pfizer's new Covid-19 vaccines that target the Omicron strains, and the nation will start a fresh booster campaign as early as next week.
The new bivalent vaccines, thought to be effective against the original Covid-19 strain and both the BA.4 and BA.5 Omicron subvariants, are the first reformulated coronavirus vaccines to be approved in the US since late 2020.
FDA commissioner Dr Robert Califf said vaccines and boosters were still saving "countless lives", while preventing hospitalisation and death from the virus.
It is expected that the US Centres for Disease Control and Prevention (CDC) will approve the new boosters after meetings this week, paving the way for immunisation to start after the Labour Day holiday weekend.
CDC data shows the US has recorded an average of 82,000 new daily cases, although that number is likely an undercount due to fewer people getting tested.
“As we head into fall and begin to spend more time indoors, we strongly encourage anyone who is eligible to consider receiving a booster dose with a bivalent Covid-19 vaccine to provide better protection against currently circulating variants," Dr Califf said.
The new boosters have not been tested on humans and the two companies are only starting their studies with results expected later in the year.
While there is not much human data on how effective the new doses are, the move is akin to how the FDA approves annual flu vaccines without clinical studies on humans.
There are concerns that the booster campaign for an additional vaccination in the US may not garner a healthy uptake, with CDC data showing less than half of the US population took up the offer of a first booster.
The group says 33.7 per cent of US adults ave had their second booster, for which only people over 50 or those at high risk with underlying conditions are eligible.