US President Donald Trump sparked an angry backlash from the medical community on Sunday with a protocol-breaking visit to his supporters outside the hospital where he is being treated for the highly infectious, potentially deadly new coronavirus.

He was masked as he waved from inside his bulletproof vehicle during the short trip outside Walter Reed military medical centre near Washington, which appeared designed to take back the narrative on his improving health after a weekend of muddled messaging from his doctors.

The last-minute limousine outing came with Trump's doctors satisfied enough about his progress to suggest the possibility of his being discharged on Monday.

But experts complained that the outing broke his own government's public health guidelines requiring patients to isolate while they are in treatment and still shedding virus — and endangered his Secret Service protection.

Trump, who has been repeatedly rebuked for flouting public health guidelines and spreading misinformation on the pandemic, said in a video that dropped on Twitter just before the appearance that he had “learned a lot about Covid” by “really going to school” as he has battled the virus.

But health experts took to the airwaves and social media to criticise the “stunt”, which they said demonstrated that he had learned nothing at all.

“Every single person in the vehicle during that completely unnecessary presidential 'drive-by' just now has to be quarantined for 14 days,” said James Phillips, chief of disaster medicine at George Washington University.

“They might get sick. They may die. For political theatre. Commanded by Trump to put their lives at risk for theatre. This is insanity.”

White House spokesman Judd Deere said “appropriate” precautions had been taken to protect Trump and his support staff, including protective gear.

“The movement was cleared by the medical team as safe to do,” he added.

But Zeke Emanuel, chair of the Department of Medical Ethics and Health Policy at the University of Pennsylvania and regular TV pundit, described the appearance as “shameful”.

“Making his Secret Service agents drive with a Covid-19 patient, with windows up no less, put them needlessly at risk for infection. And for what? A PR stunt,” he tweeted.

Confused messaging

The episode came hours after a briefing by Trump's medical team, who said he had “continued to improve” and could be returned to the White House, which has the facilities to treat and isolate the president, as early as Monday.

The president was flown to Walter Reed with a high fever on Friday after a “rapid progression” of his illness, with his oxygen levels dropping worryingly low, Trump's physician Sean Conley said in a Sunday briefing.

Health experts have complained that the messaging from the administration — and particularly Trump's medical team — has caused widespread confusion.

Conley admitted on Sunday that he had kept from the public the fact that the president had been given extra oxygen, in a bid to reflect an “upbeat attitude”.

And he gave a rosy account of Trump's progress on Saturday, only for White House chief of staff Mark Meadows to tell reporters immediately after that Trump's condition had been “very concerning” and that he was “still not on a clear path to a full recovery”.

'White House Cluster'

With his tough reelection campaign in its final month against Democratic rival Joe Biden, Trump's diagnosis and hospitalisation have left him sidelined from what he does best — campaigning.

Meanwhile, Biden — who announced on Sunday his latest negative test for the virus — will start the week with a trip on Monday to key swing state Florida.

But Trump and his advisers have done their best to project a sense of continuity.

His deputy campaign manager Jason Miller told ABC on Sunday he had spoken to Trump for a half and hour on Saturday and that the president was “cracking jokes”.

But controversy has been mounting over the possibility that Trump might have exposed numerous others to Covid-19 even after a close aide tested positive.

A timeline provided by his advisers and doctors suggested he met more than 30 donors on Thursday in Bedminster, New Jersey, even after learning that Hope Hicks had the virus — and just hours before he announced his own positive test.

There were more than 200 people at the fundraiser, and a contact tracing operation under way in New Jersey was looking at potentially thousands of people who may have been exposed.

All this came in a week when a Wall Street Journal/NBC poll — taken in the two days after a bruising presidential debate with Biden but before news emerged of Trump's illness — gave Biden a significant 53-39 per cent lead among registered voters.

As well as Trump and Hicks, numerous White House insiders and at least three Republican senators have contracted Covid-19, along with First Lady Melania Trump, who has not experienced severe symptoms.

Public health experts have expressed alarm at the “White House cluster” that has been linked to the September 26 Rose Garden celebration of conservative judge Amy Coney Barrett's nomination to the Supreme Court.