US Vice President Mike Pence did not wear a face mask during a Tuesday visit to the Mayo Clinic, violating the prestigious medical center's policy 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.
The Minnesota-based center'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."
The Centers for Disease Control and Prevention (CDC) recently updated its advice to recommend that people wear cloth face coverings in public settings where it is difficult to maintain adequate physical distance at all times, such as in grocery stores and pharmacies.
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.

Vice President Mike Pence visits the Mayo Clinic Tuesday, 28 April, 2020, in Rochester, Minnesota. Source: AAP
The vice president said he did not wear a mask because the 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.
It comes as the United States' death toll from the novel coronavirus has exceeded the 58,220 American lives lost during the Vietnam War as cases topped one million, according to a Reuters tally.
US cases have doubled in 18 days and make up one-third of all infections in the world, according to the tally.
The actual number of cases is thought to be higher, with state public health officials cautioning that shortages of trained workers and materials have limited testing capacity.
People in Australia must stay at least 1.5 metres away from others and gatherings are limited to two people unless you are with your family or household.
If you believe you may have contracted the virus, call your doctor (don’t visit) or contact the national Coronavirus Health Information Hotline on 1800 020 080. If you are struggling to breathe or experiencing a medical emergency, call 000.
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