Dr Deborah Birx, one of the leaders of the White House task force, was asked about how many Americans may be asymptomatic with Covid-19 - she said this was a "very big question".

Dr Birx said they were working on creating tests to track this and to figure out who might have resistance to the virus now.

"We want tests to be like what we use for HIV, Malaria," Dr Birx said. "That's our dream assay because it's a finger prick."

Dr Birx added that they have asked the military to test all the sailors on board the Roosevelt aircraft carrier, which is seeing a Covid-19 outbreak, in order to get at "this critical issue".

"If the first responders already knew they had it and had protective antibodies ...then it's a different dialogue," she said.

"I just want to make clear there's 150-plus countries working on this collectively together. It's devastating for every single country. When we get through this, we can go back and look at what happened, where, and what does this epidemic look like."