President Biden tested positive for COVID-19 on Thursday, the White House announced.

Driving the news: Biden's symptoms have improved after his first full day taking Pfizer's antiviral pill Paxlovid, his physician said in a letter on Friday.

• He will continue to isolate in accordance with CDC guidelines, his doctor Kevin O'Connor said in the letter.

The big picture: Biden, who is fully vaccinated and received a second booster shot in March, has "very mild" symptoms, according to a statement from White House press secretary Karine Jean-Pierre and the president. Biden said in a video post later Thursday that he's "doing well, getting a lot of work done."

• The 79-year-old president is continuing to work remotely while isolating at the White House and will "carry out all of his duties fully during that time," Jean-Pierre added. His last negative test was taken on Tuesday.

For the record: Vice President Kamala Harris tested negative and last saw Biden on Tuesday, a White House official said.

• The White House canceled Biden's trip to Wilkes-Barre, Pennsylvania, on Thursday and all other travel for at least five days.
• He recently returned from a trip to the Middle East that included stops in Israel, Palestine and Saudi Arabia and traveled to Massachusetts on Wednesday for an event on climate change.

Between the lines: White House officials are following a detailed plan that has been on the shelf for months, preparing for this exact scenario, according to Biden aides.

• They are determining who might be deemed a "close contact" for the president and who may have to consequently quarantine.
• Biden's virus sample has been sent for sequencing to determine what COVID variant he has, said Ashish Jha, the White House's COVID-19 response coordinator, at a briefing Thursday afternoon.
• It's not clear where exactly Biden was infected, Jha added.
• To minimize the disruption of potential exposures, officials wear masks in meetings and separate themselves by six feet. Those protocols are more difficult to follow on Air Force One, which Biden traveled on Wednesday.
• Staff wore masks on the plane to and from the Middle East last week.

What they're saying: "Consistent with White House protocol for positive COVID cases, which goes above and beyond CDC guidance, he will continue to work in isolation until he tests negative. Once he tests negative, he will return to in-person work," Jean-Pierre said in the statement.

• The White House will provide daily updates for transparency, she said.
• "Because the President is fully vaccinated, double boosted, his risk of serious illness is dramatically lower. He's also getting treated with a very powerful antiviral and that further reduces his risk of serious illness," Jha said at the briefing.

White House physician Kevin O'Connor said in a letter that Biden's symptoms include "mostly rhinorrhea (or 'runny nose') and fatigue, with an occasional dry cough, which started yesterday evening."

What is Paxlovid?

Paxlovid is authorized to treat those who are at high risk of developing severe COVID, including cases that lead to hospitalization or death, Axios' Oriana Gonzalez writes.

• The CDC says that adults aged 50 and older, like Biden, are more likely to get severely ill with the virus.
• Pfizer said in December that Paxlovid reduced the risk of hospitalization or death among high-risk adults by 89%.

Don't forget: In addition to older adults, the CDC considers people to be at high risk if they have underlying medical conditions, such as cancer, kidney disease, lung disease and diabetes, among others.

• Other high-risk individuals include people who are more likely to be exposed to the virus due to their work and those who cannot access health care.

How it works: The pill is most effective when taken early in the course of the infection. The FDA says patients should take Paxlovid "as soon as possible after diagnosis of COVID-19 and within five days of symptom onset."

• The treatment is administered as three tablets that should be taken orally twice a day for five days.

Editor's note: This story has been updated with new details throughout.