The US Food and Drug Administration has authorised booster doses of COVID-19 vaccines for all adults who were vaccinated with the Pfizer-BioNTech shot and the Moderna shot.

The regulator's decision paves the way for millions in the US to get additional protection and is aimed at addressing waning protection among fully vaccinated people in the face of Delta variant-driven breakthrough cases of the illness.

The FDA authorisation paves the way for all adults who had their second dose of Pfizer-BioNTech or Moderna six months prior, as well as those who had the Johnson & Johnson shot two months ago, to get a booster dose in the US.

The final regulatory review moves to a meeting of an advisory committee to the Centers for US Disease Control and Prevention (CDC) on boosters scheduled for noon on Friday and then a recommendation from the agency's director, who has publicly supported boosters for all.

More than 31 million people in the US have already received a booster shot but current CDC guidelines recommend extra doses only for some population groups.

A broad recommendation could bring uniformity in the country at a time when individual states such as Colorado, California and Massachusetts have already made all adults eligible for COVID-19 vaccine boosters.

"The messaging has become muddled," Dr Leana Wen, an emergency physician and public health professor at George Washington University, said in a post on Twitter.

Pfizer previously requested authorisation for all people aged 16 or over but US health advisers held back on extending boosters to everyone, partly because there was not enough evidence of waning immunity or the need in younger people.

Some of the CDC's outside advisers had raised concerns that booster doses are not needed in young, healthy individuals for whom the initial vaccination provides sufficient protection from serious disease and death.

Nearly 60 per cent of adults in the US- about 195.6 million people - are considered fully vaccinated, having received two doses of either the Pfizer-BioNTech or Moderna vaccines or one shot of the Johnson & Johnson vaccine.