US National Security Advisor Robert C. O'Brien File photo: Xinhua

US President Donald Trump's national Security Advisor tested positive for COVID-19 Monday, as the president prepared to visit a North Carolina facility where one of the leading vaccine candidates is being manufactured.

Trump has placed his hopes in the emergence of a successful vaccine by this fall to both contain the country's still raging coronavirus outbreak and revive his faltering election hopes.

National Security Advisor Robert O'Brien is the latest and most senior White House aide to contract the virus.

He "has been self-isolating and working from a secure location off site," the White House said in a statement, adding that there was "no risk of exposure to the president or the vice president."

Others who have been previously infected include Vice President Mike Pence's spokeswoman Katie Miller, and Kimberly Guilfoyle, a top fundraiser for the Trump campaign and girlfriend of Donald Trump Jr.

The US is by far the worst-hit country in the world, with more than 4.2 million confirmed cases and nearly 150,000 deaths.

Though the number of new cases has leveled off for the past few days, the US has so far failed to push its curve down, as other nations have done through lockdowns and physical distancing.

Trump's chief of staff Mark Meadows articulated the White House's strategy to ABC News on Sunday, telling the channel that masks weren't the answer.

"It's not masks. It's not shutting down the economy. Hopefully, it is American ingenuity that will allow for therapies and vaccines to ultimately conquer this," he said.

The US has spent more than $6.3 billion since July to fund vaccine development efforts under Operation Warp Speed.

Leading contenders include established pharmaceuticals like Johnson & Johnson, Pfizer and AstraZeneca, as well as upstarts like Novavax and Moderna.

The government is buying up stocks of their vaccines even before they are proven to work or be effective, in order to remove financial risk for the firms involved and speed up their delivery.

Operation Warp Speed aims to deliver 300 million doses of a vaccine by January 2021 - and Trump has made no secret of the fact that his goal is to immunize America first.

This is far from the rhetoric coming from European officials, who have called for a vaccine to be a "global public good."

AFP