Vice President Mike Pence is self-isolating after an aide tested positive for the coronavirus last week, joining three of the nation's top scientists in taking protective steps after possible exposure, in another stark reminder that not even one of the nation’s most secure buildings is immune from the virus.

An administration official said Pence is voluntarily limiting his exposure to other people. He has repeatedly tested negative for COVID-19 since his exposure but is following the advice of medical officials.

“Vice President Pence will continue to follow the advice of the White House Medical Unit and is not in quarantine," said spokesman Devin O'Malley.

"Additionally, Vice President Pence has tested negative every single day and plans to be at the White House tomorrow.”

Dr Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases and a leading member of the White House coronavirus task force, placed himself in quarantine over the weekend, along with Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, and the commissioner of the Food and Drug Administration, Dr Stephen Hahn.

Pence’s press secretary tested positive for the coronavirus on Friday, making her the second person who works at the White House complex known to test positive for the virus this week.

Pence has led the White House coronavirus task force for more than two months.

White House officials had confirmed Thursday that a member of the military serving as one of Trump’s valets had tested positive for COVID-19 on Wednesday.

President Donald Trump, who publicly identified the affected Pence aide as spokeswoman Katie Miller, said he was “not worried” about the virus spreading in the White House.

Nonetheless, officials said they were stepping up safety protocols for the complex.

Pence was informed of the positive test Friday morning before he left Washington for a day-trip to Iowa.

Fauci's institute said that he has tested negative for COVID-19 and will continue to be tested regularly. It added that he is considered at “relatively low risk” based on the degree of his exposure, and that he would be “taking appropriate precautions" to mitigate the risk to personal contacts while still carrying out his duties.

While he will stay at home and telework, Fauci will go to the White House if called and take every precaution, the institute said.

Redfield will be “teleworking for the next two weeks" after it was determined he had a “low risk exposure" to a person at the White House, the CDC said in a statement Saturday evening.

The statement said he felt fine and has no symptoms.

Just a few hours earlier, the Food and Drug Administration confirmed that Hahn had come in contact with someone who tested positive and was in self-quarantine for the next two weeks.

He tested negative for the virus.

All three men are scheduled to testify before a Senate committee on Tuesday. Sen. Lamar Alexander, R-Tenn., the chairman of the panel, said the White House will allow Redfield and Hahn to testify by videoconference, a one-time exception to the administration's policies on hearing testimony.

The statement was issued before Fauci's quarantine was announced.

Miller had been in recent contact with Pence but not with the president and had tested negative a day earlier.

She is married to Stephen Miller, a top Trump adviser.

The White House had no immediate comment on whether Stephen Miller had been tested or if he was still working in the White House.

The CDC and FDA would not disclose the identity of the person who had tested positive and with whom the agency leaders had come in contact.

Redfield sought to use the exposure as a teachable moment. The CDC statement said if he must go to the White House to fulfill any responsibilities as part of the coronavirus task force, he will follow CDC practices for critical infrastructure workers.

Those guidelines call for Redfield and anyone working on the task force to have their temperature taken and screened for symptoms each day, wear a face covering, and distance themselves from others.

Trump has resisted wearing a mask, and in a meeting with the nation’s top military leaders Saturday evening, he did not wear a mask during the brief portion that reporters were allowed to view.

The generals around Trump also did not wear a mask, but participants did sit a few feet away from each other.

FDA spokeswoman Stephanie Caccomo said Hahn tested negative for the virus after he learned of the contact.

He wrote a note to staff on Friday to alert them.

Six people who had been in contact with Miller were scheduled to fly with Pence on Friday to Des Moines, Iowa, on Air Force Two.

They were removed from the flight just before it took off, according to a senior administration official.

None of those people was exhibiting symptoms, but all were asked to deplane so they could be tested “out of an abundance of caution,” a senior administration official told reporters traveling with Pence.

All six later tested negative, the White House said.

The official said staff in the West Wing are tested regularly but much of Pence's staff — which works next door in the Executive Office Building — are tested less frequently. Katie Miller was not on the plane and had not been scheduled to be on the trip.

Pence, who is tested on a regular basis, was tested Friday. Miller tweeted she was “doing well” and looked forward to getting back to work.

White House chief of staff Mark Meadows said the administration was stepping up mitigation efforts already recommended by public health experts and taking other unspecified precautions to ensure the safety of the president.

He said the White House was “probably the safest place that you can come,” but the he was reviewing further steps to keep Trump and Pence safe.

The White House requires daily temperature checks of anyone who enters the White House complex and has encouraged social distancing among those working in the building.

The administration has also directed regular deep cleaning of all work spaces. Anyone who comes in close proximity to the president and vice president is tested daily for COVID-19.

Trump's valet's case marked the first known instance where a person who has come in close proximity to the president has tested positive since several people present at his private Florida club were diagnosed with COVID-19 in early March.