US Vice-President Mike Pence has visited a top US hospital without wearing a mask, despite the medical centre's own rules that visitors wear personal protective equipment.
Mr Pence appeared to be the only person present not in a facial covering at the Mayo Clinic in Minnesota.
In a deleted tweet, the clinic said the vice-president had been notified in advance of its policy requiring masks.
Mr Pence leads the White House coronavirus taskforce.
He defended his decision as necessary in order to meet with staff and patients.
The US government's own coronavirus-prevention advice is that people should wear face coverings "in public settings where other social distancing measures are difficult to maintain".
The Mayo Clinic itself requires all patients and visitors to its medical centres to wear a mask or face covering.
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Mr Pence was accompanied by commissioner of the US Food and Drug Administration Dr Stephen Hahn, who wore a mask for the trip.
The visit came on the same day that the US coronavirus caseload topped one million and the number of deaths surpassed 57,000.
Asked by a reporter on Tuesday to explain why he did not wear a mask, the vice-president said he and everyone around him are regularly tested for the coronavirus.
"When the CDC issued guidelines about wearing a mask, it was their recognition that people that may have the coronavirus could prevent the possibility of conveying the virus to someone else by wearing a mask," he said.
"And since I don't have the coronavirus, I thought it would be a good opportunity for me to be here, to be able to speak to these researchers, these incredible healthcare personnel, and look them in the eye and say thank you."
Mr Pence chose to forgo a mask earlier this month while greeting Colorado Governor Jared Polis - who did don a facial covering - on their way to the Air Force Graduation ceremony.
According to news website Axios, Mr Pence has never once worn a mask while in public since the pandemic began while continuing to travel throughout the country.
An aide to Mr Pence was the first White House member to test positive for the virus in late March.
President Donald Trump has himself previously said he has no plans to wear a mask.
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