The US Centers for Disease Control and Prevention (CDC) has recommended Moderna and Johnson & Johnson's Covid booster doses and has given the freedom for people to mix and match the third shot.
"Eligible individuals may choose which vaccine they receive as a booster dose. Some people may have a preference for the vaccine type that they originally received and others may prefer to get a different booster. CDC's recommendations now allow for this type of mix and match dosing for booster shots," the agency said in a statement late on Thursday.
The boosters were unanimously recommended by the CDC during its vaccine advisory's two-day meeting held between October 20-21.
It was approved by the Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee last week.
Moderna's booster shots were recommended for elderly people and at-risk adults six months after they complete their primary series of shots, similar to Pfizera's booster.
But unlike Pfizer Moderna has been recommended for only half-dose booster to bolster protection against the virus.
The agency also endorsed J&J boosters for everyone 18 and older who received the initial shot at least two months ago.
"These recommendations are another example of our fundamental commitment to protect as many people as possible from Covid-19. The evidence shows that all three Covid-19 vaccines authorised in the US are safe -- as demonstrated by the over 400 million vaccine doses already given," said Rochelle P. Walensky, CDC Director, in the statement.
"And, they are all highly effective in reducing the risk of severe disease, hospitalization, and death, even in the midst of the widely circulating Delta variant," Walensky added.
The 'mix and match' approach may help nearly 15 million people who got the Johnson & Johnson Covid-19 vaccine and more than 39 million Moderna recipients. It will also provide flexibility to doctors and other vaccinators.
While the approval makes millions of people eligible to receive a booster shot and to benefit from additional protection, "it should not distract from the critical work of ensuring that unvaccinated people take the first step and get an initial Covid-19 vaccine", the statement said.
According to the CDC, more than 65 million Americans remain unvaccinated, making them vulnerable for the infectious disease.
Three vaccines have been to date authorised for use against Covid in the US: Pfizer, Moderna and Johnson & Johnson.
However, several studies have pointed out that the effectiveness of the two-dose vaccine against infection wanes over time, stressing the need for booster doses.