Donald Trumps 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 countrys still raging coronavirus outbreak and revive his faltering election hopes. National security advisor Robert OBrien 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 Pences spokeswoman Katie Miller, and Kimberly Guilfoyle, a top fundraiser for the Trump campaign and girlfriend of Donald Trump Jr. The United States 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. Trumps chief of staff Mark Meadows articulated the White Houses strategy to ABC News on Sunday, telling the channel that masks werent the answer. "Its not masks. Its not shutting down the economy. Hopefully, it is American ingenuity that will allow for therapies and vaccines to ultimately conquer this," he said. Trump will visit Fujifilm Diosynth Biotechnologies in Morrisville, North Carolina on Monday afternoon, which has just been awarded a contract to manufacture Novavaxs experimental vaccine. 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." Last week, the US signed a $1.95 billion deal with US pharma giant Pfizer and Germanys BioNTech for 100 million doses of their vaccine, the biggest deal to date. "Hopefully the approval process will go very quickly, and we think we have a winner there," Trump said. "We also think we have other companies right behind that are doing very well on the vaccines, long ahead of schedule." Results by November? Moderna, one of vaccine makers that is furthest ahead in its development timeline, began the final stage of its clinical trial Monday in collaboration with the National Institutes of Health. The trial will be conducted across the United States and is expected to enroll approximately 30,000 adult volunteers who do not have COVID-19. Participants will get two doses of the vaccine given 28 days apart. A notable irony of the process is that, given the rampant nature of the US epidemic, results could come far sooner than the two years the trial is set to run. Daily new cases have plateaued around the 66,000 mark for the past few days. "Its a really optimistic scenario. It could be November," CEO Stephane Bancel told CNBC on Monday. "Again, at this stage, its impossible for us to know precisely. It will depend on the event rate, the attack rate of infection."