WASHINGTON (REUTERS, AFP) - The US government on Saturday (Feb 27) authorised Johnson & Johnson's single-dose Covid-19 vaccine, enabling millions more Americans to be vaccinated in the coming weeks and setting it up for additional approvals around the world.

The US Food and Drug Administration (FDA) announced the emergency use authorisation for adults aged 18 and older following Friday's unanimous endorsement by the agency's panel of outside experts.

"The authorisation of this vaccine expands the availability of vaccines, the best medical prevention method for Covid-19, to help us in the fight against this pandemic, which has claimed over half a million lives in the United States," Acting FDA commissioner Janet Woodcock said in a statement.

In J&J's 44,000-person global trial, the vaccine was found to be 66 per cent effective at preventing moderate-to-severe Covid-19 four weeks after inoculation, and 100 per cent effective in preventing hospitalisation and death due to the virus.

There were very few serious side effects reported in the trial, which also offered some preliminary evidence that the vaccine reduced asymptomatic infections.

However, more study is expected, and the FDA on Sunday dismissed the idea that evidence proved the vaccine prevented transmission between people and added there was no data to determine how long the vaccine's protection lasted.

Us President Joe Biden cheered the announcement but cautioned Americans to not let their guard down as new variants spread.

"Though we celebrate today's news, I urge all Americans - keep washing your hands, stay socially distanced, and keep wearing masks," Mr Biden said in a statement. "As I have said many times, things are still likely to get worse again as new variants spread, and the current improvement could reverse."

J&J's vaccine is expected to be used widely around the globe because it can be shipped and stored at normal refrigerator temperatures, making distribution easier than for the Pfizer-BioNTech and Moderna vaccines, which are shipped frozen.

"It potentially could play a very substantial role if we have enough doses because it's only a single-dose vaccine and that will make it attractive to people who are difficult to reach," said Dr William Schaffner, an infectious disease expert at Vanderbilt University Medical Centre in Nashville. "It's one and done."

Dr Schaffner said the fact that the J&J vaccine can be stored at normal refrigeration temperatures will make it easier to distribute. "We can bring the vaccine to the people," he said.

The US government, which has purchased 100 million doses of the J&J vaccine, plans to distribute about three million to four million this week. That would be on top of the around 16 million doses of the Pfizer-BioNTech and Moderna vaccines it already planned to ship across the country.

J&J plans to provide a total of 20 million doses by the end of March, which along with the more than 220 million total doses expected from Pfizer-BioNTech and Moderna would be enough to fully vaccinate 130 million people in the United States.

So far, the US has distributed more than 90 million doses of Covid-19 vaccines, some of which have been used for second shots. About 14 per cent of Americans have received at least one dose, according to US government data.

Covid-19 has claimed more than half a million lives in the US and states are clamouring for more doses to stem cases, hospitalisations, and deaths.

Public health officials have warned about a rise in the prevalence of more contagious variants of the virus, adding to the urgency to get millions more people vaccinated as quickly as possible. Meanwhile, they said, mask wearing and other measures to curb the virus spread should remain in place as recent declines in Covid-19 cases appear to be levelling off.

The J&J vaccine is also under review by the European Union, where deliveries are expected starting in April and would build on the region's thin supplies of the Pfizer-BioNTech, Moderna and AstraZeneca shots.

In South Africa, regulators were awaiting the FDA decision as their government looks to deploy more J&J vaccine against a variant of the virus called B.1.351 that is able to evade some vaccine protection.

J&J's vaccine is being rolled out there prior to official authorisation for about 500,000 healthcare workers as it tries to stem infections from the variant, which has swept across the country and spread globally, including to the US.

The vaccine is one of the few that has been tested in clinical trials against the variant and had a 64 per cent efficacy rate at preventing moderate-to-severe disease in South Africa.

J&J said on Friday that the company was developing a second-generation vaccine that would target the concerning South African variant and will be ready to start Phase I trials by this summer.

J&J's vaccine uses a common cold virus known as adenovirus type 26 to introduce coronavirus proteins into cells in the body and trigger an immune response.

Both the Pfizer-BioNTech and Moderna vaccines, which are based on new messenger RNA technology, showed higher efficacy rates in their pivotal trials when tested using two doses versus J&J's single-shot vaccine. But experts cautioned against close comparisons because the trials had different goals and J&J's was conducted while the more contagious new variants of the virus were circulating.

J&J is testing a two-dose version of its vaccine, with results expected this summer.