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Medical experts are questioning Donald Trump’s decision to greet supporters in a drive-past outside the hospital where he is being treated for Covid-19.
There are concerns the US president, who wore a mask, may have endangered Secret Service staff inside the car.
White House spokesperson Judd Deere said the trip on Sunday had been “cleared by the medical team as safe”.
Questions remain over the seriousness of Mr Trump’s illness after conflicting statements over the weekend.
The president has been in hospital since Friday, after he announced hours earlier that he had tested positive for the virus.
Covid-19 has infected more than 7.4 million and killed 200,000 people across the US, according to Johns Hopkins University.
President Trump’s diagnosis has upended his election campaign, as he faces Democratic challenger Joe Biden on 3 November.
What did the president do?
Mr Trump waved to well-wishers from behind the glass of a sealed car after tweeting that he would leave Walter Reed hospital, near Washington, to pay a “surprise visit” to “patriots” outside.
Experts say the president’s short car trip broke public health advice to quarantine when seeking treatment for the virus, and may have put Secret Service agents inside the vehicle at risk of infection.
“That Presidential SUV is not only bulletproof, but hermetically sealed against chemical attack. The risk of Covid-19 transmission inside is as high as it gets outside of medical procedures,” tweeted Dr James Philips, a doctor at the same hospital where the president is being treated.
He said that those inside the president’s car were at risk of getting ill and would need to quarantine for 14 days.
Democrats have also criticised the trip, with House of Representatives Hakeem Jeffries tweeting: “We need leadership. Not photo ops.”
But the White House’s Judd Deere defended the move, saying “appropriate precautions were taken in the execution of this movement to protect the president and all those supporting it, including PPE [personal protective equipment]”.
What did we learn at the weekend?
Over the weekend, it emerged the president’s condition was more serious than previously reported when he went to hospital on Friday evening.
The White House had said that Mr Trump was experiencing “mild symptoms” of Covid-19, but it was confirmed at the weekend that he received extra oxygen after his levels dipped twice in two days.
He was also given the steroid dexamethasone, which is normally reserved for serious cases, according to experts.
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There is scepticism over the prospect – raised by doctors earlier – that Donald Trump could leave hospital as early as Monday.
The president, being 74, a man and someone categorised as obese, is in a higher-risk category for Covid-19. On Friday he was given an experimental drug cocktail injection and started a five-day course of antiviral medication remdesivir.