CDC director Rochelle Walensky signed off on the recommendations late on Thursday, aligning the agency with the US Food and Drug Administration's authorisation on Wednesday for the additional boosters and "mix and match" dosing.

"There are now booster recommendations for all three available COVID-19 vaccines in the United States. Eligible individuals may choose which vaccine they receive as a booster dose," the agency said on Thursday.

That could pave the way for the extra shots to become available immediately afterward to millions of Americans.

Still, health officials and public health experts said the booster rollout could be confusing.

The panel struggled with trying to make the language of its recommendations as clear as possible, and also offer flexibility for patients to get a vaccine of their choice.

"A really important aspect of all of this is being clear and not dancing on the head of a pin, so that we don't further confuse the American people," said Dr Beth Bell, a panel member and clinical professor in the department of global health at the University of Washington in Seattle.

The recommendations also open the door for recipients of the one-shot J&J vaccine to get a dose of the Pfizer or Moderna vaccines that have been shown to afford greater protection in a variety of studies.

Although these vaccines have been highly effective in preventing serious illness and death, some government scientists have suggested boosters are needed to keep immunity high, especially as the extremely contagious Delta variant of the virus can cause breakthrough infections among some who are fully vaccinated.

ACIP voted to recommend booster doses for everyone 18 and older who received a first dose of Johnson & Johnson's vaccine at least two months earlier.

For those who received their second dose of Moderna's vaccine at least six months earlier, ACIP recommended a third shot for those age 65 and over, as well as some individuals at risk or severe illness and those at high risk of exposure to the virus through their jobs.

The FDA and CDC previously signed off on booster shots of the COVID-19 vaccine from Pfizer and partner BioNTech SE for the same groups included in the Moderna recommendations.

About 11.6 million people have received a booster dose, according to data from the CDC

The FDA and CDC have been under pressure to authorise the additional shots after the White House announced plans in August for a widespread booster campaign.

Other countries such as Israel have begun offering boosters to a broad population, but it is not yet known whether the United States will follow suit.

Australian Associated Press