Trump declares himself immune to Covid-19

US President Donald Trump has declared himself immune to Covid-19 and ready for an election fight as his White House race against surging rival Joe Biden enters its critical final weeks.

Trump's doctors gave him the all-clear on Saturday to return to the campaign trail after he was ruled no longer a coronavirus transmission risk, and on Sunday he said while calling in to a campaign event, that he tested "totally negative."

But he has yet to be medically-declared virus-free, and his immunity claim remains unproven.

Twitter on Sunday hid Trump's tweet, in which he claimed he was immune, saying the post violated its rules about misleading and potentially harmful misinformation on Covid-19.

The tweet is still visible once users click through the warning.

READ MORE: White House Dr: Trump no longer at risk of transmitting coronavirus

"It looks like I'm immune for, I don't know, maybe a long time and maybe a short time, it could be a lifetime, nobody really knows, but I'm immune," Trump told the Fox News show "Sunday Morning Futures."

"The word immunity means something, having really a protective glow.

"So now you have a president who doesn't have to hide in a basement like his opponent," Trump added, in a jab at Democratic challenger Biden, who has taken a far more cautious approach to campaigning during the pandemic.

But it is not yet clear to what degree contracting Covid-19 confers immunity, with early studies suggesting a few months while newer ones have indicated it could last longer.

Trailing in polls 

Trump, 74, was treated with an experimental antibody cocktail made by Regeneron that may give immunity for just a few months when taken as a treatment rather than as a vaccine.

"In some cases, vaccines can last for decades. (But) if you get it in the form of natural immunity, that isn't known yet," Regeneron CEO Leonard Schleifer told CBS's "Face the Nation" on Sunday.

"If you get it in our vial, if you will, that's probably going to last you for months," he said.

Badly trailing Biden with just weeks until the November 3 vote, Trump has been counting the days until he can hit the trail again.

The Republican leader rallied hundreds of supporters for a comeback event at the White House on Saturday, and is planning back-to-back rallies this week – in Florida on Monday, then Pennsylvania and Iowa – in a bid to salvage his struggling campaign.

During his phone interview on Fox News, Trump suggested that his White House rival could himself be sick.

"If you look at Joe, he was coughing yesterday horribly and grabbing his mask, as he's coughing," Trump said. "And I don't know what that was all about, and it didn't get much press."

READ MORE: Trump makes first public appearance since Covid-19 diagnosis

'Never endorsed a candidate' 

The Biden campaign has been publishing daily coronavirus tests for their 77-year-old candidate since Trump tested positive on October 1 – landing him in a military hospital for three nights and derailing his campaign.

There has been less transparency surrounding Trump's own state of health, with his medical team repeatedly refusing to say when he last tested negative for the virus.

That has fueled suspicion that he may not have been tested for several days prior to his diagnosis.

Covid-19 has killed more than 214,000 people in the United States, but the president has urged Americans not to fear the virus.