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 authorisation 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.

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