The cringe-worthy moment US President Donald Trump learns a “solid flu vaccine” won’t stop the coronavirus has been caught on video.

After days of downplaying the threat of coronavirus to the United States, the seriousness of the outbreak has finally hit home for Donald Trump.

The US President today announced he would donate part of his salary to “confront, contain, and combat” the virus, as the death toll rose from six to nine overnight, with at least 124 confirmed cases.

Mr Trump’s gesture comes just one day after a White House briefing by medical experts on SARS-CoV-2, the virus which causes the deadly COVID-19 disease, appeared to leave him momentarily stunned.

In a clip that has since been viewed millions of times online, Mr Trump unwittingly revealed his ignorance about the virus as he grilled the group, which included National Institute of Allergy and Infectious Diseases chief Dr Anthony Fauci and pharmaceutical executives.

The footage begins halfway through a conversation about the flu vaccine – which the public is being urged to get to lessen the burden on health care systems as they deal with the coronavirus.

“But the same vaccine could not work … you take a solid flu vaccine and you don’t think that would have an impact or much of an impact on corona? (sic)” Mr Trump pitches in.

“No,” one expert responds simply, rendering Mr Trump momentarily speechless as a stunned look spreads across his face.

There is a short pause as the President looks to his side for confirmation and another expert, also out of shot, delivers a more gentle: “Probably not.”

Mr Trump, arms crossed tightly to his chest, keeps nodding and repeats “probably not” under his breath while nodding vigorously.

The unseen expert says it again more loudly, “Probably not,” prompting more nodding from Mr Trump.

Virologists warned on Monday it was likely SARS-CoV-2 had been circulating undetected for weeks in Washington State, where all nine of the deaths have occurred including eight in King County (around Seattle) and one in Snohomish County, north of King County.

Local scientist Trevor Bedford and his team estimated the virus landed in Seattle around January 15, infecting several hundred people who remained undiagnosed due to lack of testing.

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Mr Bedford’s research prompted The Washington Post to ask whether the state’s largest city, with a population of 600,000, was doomed to become the new Wuhan, saying the outbreak was at “a critical juncture”.

“The genomic data support a linkage between the WA1 (first case) from January 19 and the WA2 (second case) from February 24, both from Snohomish County,” Mr Bedford tweeted in a summary of his findings.

“This suggests that the case WA1 infected someone who was missed by surveillance due to mild symptoms and a transmission chain was initiated at this point in mid-January.

“Because the case definition from mid-January until February 27 required direct travel from China, any of these subsequent cases in the transmission chain would not have been tested for #SARSCoV2.

“Running these simulations forward with a mean doubling time of 6.1 days, we estimate the number of current infections in this transmission chain on March 1 to be 570 with an 90 per cent uncertainty interval of between 80 and 1500.”

I wrote a blog post that details Saturday's finding of "cryptic transmission" of #SARSCoV2 in Washington State. Genomic evidence suggests that #COVID19 has been circulating here since Jan 15, but undetected due to lack of testing. https://t.co/cYmyTdU4Cg pic.twitter.com/5FJNwUzJ2E — Trevor Bedford (@trvrb) March 3, 2020

Running these simulations forward with a mean doubling time of 6.1 days, we estimate the number of current infections in this transmission chain on March 1 to be 570 with an 90% uncertainty interval of between 80 and 1500. 6/11 pic.twitter.com/Vm99N3x61s — Trevor Bedford (@trvrb) March 2, 2020

The US is lagging behind other less-developed countries in testing for the disease. Many US states are not equipped to test for the virus and those who do have encountered problems with faulty kits.

“The spread of the virus has gone undetected in part because many infected people experience only mild infections that could be confused for a cold or the flu, and in part because of stumbles in the Centres for Disease Control and Prevention’s effort to develop test kits for state and local public health laboratories, which has meant very little testing has been done in the country until the past few days,” the Post said.

Washington Democrat Senator Patty Murray said the failure to develop and distribute working test kits to public health agencies had lost America valuable time.

“I am hearing from people personally across our state who are frustrated,” Ms Murray said.

“They believe they have been exposed, they are sick, they want to get tested, they have nowhere to go.

“People want to be safe and protected, they do not want to spread this, they want to take care of themselves but we don’t have the capability right now to do it with the policies we have in place.”

Mr Bedford said Seattle authorities have be forced to mirror the aggressive actions taken by their counterparts in Wuhan in order to contain the virus.

The birthplace city of SARS-CoV-2 and its 11 million residents have been living in a state of quarantine since late January.