The United States Tuesday authorized a fourth dose of either the Pfizer-BioNTech or Moderna Covid-19 vaccines for people 50 and older, as authorities warn of a possible new wave driven by the BA.2 variant.

The Food and Drug Administration (FDA) said in a statement it had based its decision on emerging evidence that an additional booster, given four months after the last, improved protection against severe Covid and wasn't associated with new safety concerns.

Additionally, people with immune compromising conditions who have already received four shots, with their latest at least four months ago, are now eligible for a fifth dose.

That includes people living with certain organ transplants.

The Pfizer vaccine will be available to immune compromised people aged 12 and over, while the Moderna vaccine will be available to those 18 and up.

"Current evidence suggests some waning of protection over time against serious outcomes from Covid-19 in older and immunocompromised individuals," senior FDA scientist Peter Marks said, explaining the decision.

The FDA said data from Israel, which had studied the effects of a fourth dose given four months after the third on 700,000 people, found the extra shot was safe.

It also cited data from a study of 154 health workers whose antibody levels, including against the Delta and Omicron variants, were restored to high levels two weeks after their fourth doses.

A study published by Israeli researchers in the New England Journal of Medicine this month indicated that three doses of current generation mRNA vaccines have hit a ceiling in terms of the immune response generated.

In other words, while three doses brings immunity levels to new heights, the fourth dose restores antibody levels to where they previously were shortly after the third.

Experts have said the benefits to younger, healthy people aren't yet clear, and say new vaccines will likely need to be developed as the virus continues to mutate.