The United States is getting a third vaccine to prevent COVID-19, as the Food and Drug Administration has cleared a Johnson & Johnson shot that works with just one dose instead of two.

Health experts are anxiously awaiting a one-and-done option to help speed vaccinations, as they race against a virus that already has killed more than 510,000 people in the US and is mutating in increasingly worrisome ways.

The FDA said J&J’s vaccine offers strong protection against what matters most: serious illness, hospitalisations and death.

One dose was 85 per cent protective against the most severe COVID-19 illness, in a massive study that spanned three continents – protection that remained strong even in countries such as South Africa, where the variants of most concern are spreading.

“The more vaccines that have high efficacy that we can get into play, the better,” Dr. Anthony Fauci, the top US infectious disease expert, said ahead of the FDA’s ruling on Saturday.

Shipments of a few million doses to be divided among states could begin as early as Monday. By the end of March, J&J has said it expects to deliver 20 million doses to the US, and 100 million by the northern summer.

J&J also is seeking authorisation for emergency use of its vaccine in Europe and from the World Health Organisation.

Worldwide, the company aims to produce about one billion doses globally by the end of the year.

On Thursday, the island nation of Bahrain became the first to clear its use.

-AAP