US regulators have formally approved the single-shot Johnson & Johnson coronavirus vaccine, the third jab to be authorized in the country.

The vaccine is set to be a cost-effective alternative to the Pfizer and Moderna vaccines, and can be stored in a refrigerator instead of a freezer.

Trials found it prevented serious illness but was 66% effective overall when moderate cases were included.

The vaccine is made by the Belgian firm Janssen.

The company has agreed to provide the US with 100 million doses by the end of June. The first doses could be available to the US public as early as next week.

The UK, EU and Canada have also ordered doses, and 500 million doses have also been ordered through the Covax scheme to supply poorer nations.

President Joe Biden hailed it as "exciting news for all Americans, and an encouraging development", but warned that the "fight is far from over".

"Though we celebrate today's news, I urge all Americans - keep washing your hands, stay socially distanced, and keep wearing masks," he 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."

The authorization by the US Food and Drug Administration (FDA) came after an external committee of experts unanimously backed the vaccine on Friday.

Results from trials conducted in the US, South Africa and Brazil showed it was more than 85% effective at preventing serious illness, and 66% effective overall when moderate cases were included.

Notably, there were no deaths among participants who had received the vaccine and no hospital admissions after 28 days post-vaccine.

Overall protection was lower in South Africa and Brazil, where virus variants have become dominant, but defence against severe or critical illness was "similarly high".

South Africa began administering the unapproved Johnson & Johnson jab to healthcare workers as part of a study earlier this month. It came after early trials suggested the Oxford-AstraZeneca vaccine offered "minimal protection" against mild disease from the variant dominant in large parts of the country.

So far the only other country to approve the vaccine for emergency use is Bahrain, which gave it the green light on Thursday.

Because the vaccine will require fewer doses than its two-shot Pfizer and Moderna counterparts, it will also require fewer vaccine appointments and medical staff.

The Johnson & Johnson vaccine uses a common cold virus that has been engineered to make it harmless.

It then safely carries part of the coronavirus's genetic code into the body. This is enough for the body to recognise the threat and then learn to fight coronavirus.

This trains the body's immune system to fight coronavirus when it encounters the virus for real.

This is similar to the approach used by the University of Oxford and AstraZeneca.

What is the situation in the US?

About 72.8 million Americans have already been vaccinated and about 1.3 million doses are being administered across the country each day. President Biden has pledged to administer 100 million vaccines in his first 100 days in office.

More than 508,000 people in the US have died of Covid but new cases, hospitalisations and deaths have all been on the decline over the past few weeks.

Top public health experts, however, continue to warn that mutations of the virus can still threaten progress.