Trump's national security adviser returns to White House after coronavirus diagnosis

(CNN)National security adviser Robert O'Brien returned to the White House Tuesday after a period of self-isolation following his coronavirus diagnosis last month, a National Security Council spokesperson said.

O'Brien met with President Donald Trump "who warmly welcomed him back" to the West Wing following two negative Covid-19 tests and more than a week without symptoms, NSC spokesperson John Ullyot said in a statement.

"His return to duties at the White House is consistent with CDC guidelines and White House Medical Unit advice," Ullyot added.

O'Brien's diagnosis last month marked the highest-ranking Trump administration official known to have tested positive.

He had experienced "mild symptoms" and worked "from a secure location off site," according to an unnamed statement to the press from the White House confirming his test results last month.

Several National Security Council staffers told CNN that they weren't informed that O'Brien tested positive and learned of the news from media reports.

O'Brien, one of Trump's top aides, had recently returned from Europe, where he and his top deputy met with officials from the UK, France, Germany and Italy. He also took a family vacation, during which he was likely exposed to the virus, immediately following his trip to Europe, a senior administration official previously told CNN.

While O'Brien was able to return to the White House in a relatively short amount of time, the disruption caused by his positive test underscores the unique challenge staffers face as they try to keep the pandemic out of the West Wing while the President pushes to reopen the country.

Last month, administration officials were alerted that a cafeteria employee on White House grounds had tested positive. And Kimberly Guilfoyle, the President's son's girlfriend and top campaign fundraising official, tested positive before a trip to Mt. Rushmore in July.

Additionally, multiple Secret Service agents and campaign staffers tested positive after becoming infected while preparing for the President's Tulsa campaign rally in June. Vice President Mike Pence's communications director Katie Miller also tested positive for coronavirus in May, and one of the President's personal valets tested positive earlier that week.

As a result, the White House has since gone to great lengths to prevent Trump and Pence from contracting the virus, even as they travel to states where cases are surging.

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