President Donald Trump said Friday that the United States has authorized a second coronavirus vaccine, jumping the gun on regulators who have yet to give it the formal green light.

"Moderna vaccine overwhelmingly approved. Distribution to start immediately," he wrote on Twitter.

This follows a recommendation by an expert panel on Thursday to grant emergency use approval for Moderna's Covid-19 vaccine in the United States.

The Food and Drug Administration is expected to issue formal approval Friday. This would make Moderna's vaccine the second to be approved in a Western country following the one developed by Pfizer-BioNTech.

Trump's announcement came as US Vice President Mike Pence and his wife received the Covid-19 vaccine live on television, in a public display designed to boost national confidence in the measure.

Pence indicated that formal approval for Moderna would be a matter of hours.

"We have one, perhaps within hours two, safe and effective coronavirus vaccines," he said.

Thursday's hearing came as the number of deaths from the coronavirus quickly approaches 310,000 in the country with the world's biggest Covid-19 outbreak, which this week began vaccinating health care workers and long-term care residents with the Pfizer vaccine.

Both of the frontrunner vaccines are based on cutting-edge mRNA (messenger ribonucleic acid) technology, which had never been approved prior to the pandemic, and both are two-dose regimens. 

The US, which has recorded more than 17 million cases of the virus, will probably become the first country to approve the Moderna vaccine.