A hot microphone left on in the White House press briefing room has sparked controversy, after a journalist was overheard joking that everyone there had been vaccinated against the coronavirus.

Two journalists - identified online  as Fox News' John Roberts and New York Times photographer Doug Mills - were caught chatting before an official press briefing. 

'You can take off the mask, the case fatality rate is 0.1 to 0.3 according to USC,' Roberts is heard saying.

Mills then apparently jokes saying, 'We've all been vaccinated around here.' 

The video has since gone viral - and has been jumped on by conspiracy theorists. There is currently no vaccine or cure for coronavirus, which has killed more than 40,000 Americans, and infected more than 800,000.

When White House briefings are streamed live, some outlets often carry the feeds in advance complete with audio and video before the events begin. 

 Roberts is also heard quoting statistics, saying: 'USC and LA County public health came out with a study that found that there are 7,000 cases in California but they really believe there are anywhere from 221,000 to 442,000 people who are infected.' 

'So it was a hoax,' Mills jokes. '

But Roberts replies: 'No, I don't think it was a hoax'.

Roberts appears to be referring to a study this week which found four percent of adults in Los Angeles County tested positive for COVID-19 antibodies.

It suggests hundreds of thousands of people might have actually been infected in early April when only 8,000 cases had been confirmed - meaning many people may have the virus but show no symptoms. The antibody testing was carried out last week by county officials working with scientists from USC. The test is manufactured by Premier Biotech and has a 90-95% accuracy rate.

They randomly tested 863 adults with finger prick tests. Of those, 4.1 percent tested positive for the antibodies.

 When scientists adjusted the results to factor in a margin for error, they said that between 2.8 percent and 5.6 percent of the county's adult population - between 221,000 and 442,000 people - had the virus.

During the briefing that followed, Roberts went on to ask Dr. Deborah Birx on the White House coronavirus task force about the study he mentioned to Mills.  

Dr. Birx explained that the study was of interest because there is no specific data about the number of asymptomatic cases which would be crucial to understanding the infectious rate and the mortality rate. 

She went on to explain that the team is looking at the findings of the study which might see authorities to 'recalibrating disease prediction models and rethink public health strategies.' 

Mediaite was first to report on the viral video. On social media some tweeted that they believed the video might have been faked while others have said that Mills was likely not joking. 

Some suggested the exchange proved that coronavirus is 'fake news'.

'Seems like an intentional leak. Well played if so. Wake up another wave,' wrote one. 

'WTF does the guy in the mask mean by "everybody here has been vaccinated anyway?"' questioned another. 

'Well does that mean they have a COVID vaccine? ?? I don't want it anyway but sure seems shady if so,' tweet another user.

Social media users were completely divided on whether the conversation was a joke or for real.