Moderna has been approved by the US government as the country's second Covid-19 vaccine, clearing the way for millions of doses to be released.
The Food and Drug Administration (FDA) authorised the US-made jab about a week after approving a Pfizer/BioNTech vaccine which is now being distributed.
The US has agreed to purchase 200 million doses of Moderna, and six million may be ready to ship now.
The country has the world's highest numbers of Covid-19 deaths and cases.
It has recorded at least 311,529 deaths and 17,269,542 infections , according to Johns Hopkins University.
"To go from having a sequence of a virus in January to having two vaccines available in December is a remarkable achievement," said Dr James Hildreth, CEO of Meharry Medical College in Tennessee.
An FDA advisory panel on Thursday voted 20-0 with one abstention that the benefits of the Moderna vaccine outweighed the risks for those aged 18 and over, and authorisation followed on Friday.
Regulators reported earlier this week that the Moderna vaccine was safe and 94% effective .
Hours before the official announcement, President Donald Trump tweeted that the vaccine had been "overwhelmingly approved" and distribution would "start immediately".
BBC
