US Vice President Mike Pence chose not to wear a face mask during a visit to the Mayo Clinic in Minnesota, violating the prestigious medical centre's policy and despite his team being warned in advance.
Video showed Mr Pence visiting staff and a patient in a crowded hospital room without wearing a mask and he appeared to be the only person present not wearing one.
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The centre's policy states: "Part of our protocol for ensuring your safety is to require all patients, visitors and staff to wear a face covering or mask while at Mayo Clinic to guard against transmission of Covid-19."
The clinic said in a statement that it had "shared the masking policy with the VP's office".
Mr Pence leads the US government's coronavirus taskforce.
He said he did not wear a mask because the US Centers for Disease Control and Prevention (CDC) guidelines suggest that masks help prevent the spread of the virus by those who are infected - and he is not.
"Since I don't have the coronavirus, I thought it'd be a good opportunity for me to be here, to be able to speak to these researchers, these incredible health care personnel, and look them in the eye and say thank you," he said.
The CDC recently updated its advice to recommend that people wear face coverings in public settings where it is difficult to maintain adequate physical distance at all times.
This is because a very high proportion of people infected with the coronavirus are believed to be asymptomatic and because recent research has shown that it may in certain circumstances be possible to infect others through normal speaking and breathing.
Mr Pence, President Donald Trump, and other senior White House officials have pointedly ignored the advice, leading to speculation it is a coordinated decision to downplay the severity of the crisis.
The virus has killed 58,365 people in the United States to date.
