President Donald Trump of the United States of America made a surprise public appearance in a drive-past to greet supporters despite still being infected with the novel coronavirus (COVID-19).

Trump was photographed wearing a mask and waving to supporters outside the Walter Reed Military Medical Centre, Maryland, in the presidential vehicle while members of the secret service with him wore medical-grade masks and personal protective equipment (PPE).

The act, however, drew criticism from medical experts.

A doctor at the hospital strongly criticised the president’s motorcade ride due to the infection risk inside the car.

“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,” Dr James Philips tweeted.

He said that those inside the president’s car would need to quarantine for 14 days and are at risk of getting ill.

Also, the opposition Democratic Party lambasted the president, insisting that photo-ops were not needed at such a critical period.

“More than 205,000 Americans are dead. We need leadership. Not photo ops,” tweeted Hakeem Jeffries, Chairman of the Democrats in the US House of Representatives.

Earlier, it emerged that the president’s condition was more serious than previously reported when he went to the hospital Friday evening.

The White House had said that Trump was experiencing “mild symptoms” of COVID-19, but it was confirmed at the weekend that he received supplemental oxygen after his levels dipped twice within two days.

He was given the steroid dexamethasone, which is normally reserved for serious cases, according to experts.

However, Trump sought to project an image of strength, tweeting images of himself appearing to work from the hospital and a video message teasing his surprise drive past.

In a tweet, the president – dressed in a suit jacket and shirt with no tie – said: “I learned a lot about Covid. I learned it by really going to school. This is the real school. This isn’t the let’s read the books school. I get it and I understand it. It’s a very interesting thing, I’ll be letting you know about it.”

The president’s diagnosis, which he made public in a tweet early on Friday, has upended his election campaign. Mr Trump faces Democratic challenger, Joe Biden, on 3 November.