WASHINGTON — President Joe Biden tested positive for COVID-19 on Thursday and is isolating with "very mild symptoms," the White House said, as new variants of the highly contagious virus challenge the nation's efforts to get back to normal after two and a half years of pandemic.

White House Press Secretary Karine Jean-Pierre said Biden had begun taking Paxlovid, an antiviral drug designed to reduce the severity of the disease. He was isolating in the family quarters of the White House and "continuing to carry out all of his duties fully," she said.

Biden's physician, Dr. Kevin O'Connor, said in a letter that Biden had a runny nose and "fatigue, with an occasional dry cough, which started yesterday evening."

Biden himself said in a video posted on Twitter: "I really appreciate your inquiries and concerns. But I'm doing well, getting a lot of work done."

Biden, 79, is fully vaccinated, after getting two doses of the Pfizer coronavirus vaccine shortly before taking office, a first booster shot in September and an additional dose March 30. The president will isolate for five days and can return to his usual activities after a negative test, White House COVID-19 coordinator Dr. Ashish Jha told reporters at a briefing.

Jean-Pierre described the president's symptoms as “very mild” and said Biden had been in contact with members of the White House staff by phone and would participate in his planned meetings "via phone and Zoom from the residence."

Asked where Biden might have contracted the virus, Jean-Pierre said, "I don't think that matters." She later clarified that to say the White House was focused on how Biden was feeling and would engage in contact tracing.

In the video that Biden recorded outdoors to tell the world he was OK, the videographer stood six feet away and wore an N95 mask, Jean-Pierre said. The president will stop taking his blood thinner and cholesterol medications while receiving Paxlovid.

The White House took steps to show that the president was busy working despite his diagnosis, with Biden tweeting out a picture of himself making calls from the treaty room of the White House.

The president spoke by phone to lawmakers in Pennsylvania to apologize for having to cancel his planned trip Thursday to the city of Wilkes-Barre to promote his crime prevention plans. Biden also called South Carolina Rep. Jim Clyburn to wish him a happy birthday and congratulate him on receiving an award from the NAACP.

Dr. O'Connor wrote in his letter about the president's treatment plan: "I anticipate that he will respond favorably" to Paxlovid "as most maximally protected patients do."

Jean-Pierre said Biden had tested negative on Tuesday and would stay isolated until he tests negative again.

White House chief of staff Ron Klain said in a letter to White House staff obtained by The Associated Press that "all close contacts of the president" will be informed of Biden's positive test "per standard protocol."

First lady Jill Biden, speaking to reporters as she arrived for a school visit in Detroit, said she'd just gotten off the phone with her husband.

"He's doing fine," she said. "He's feeling good."

The first lady, who was wearing a mask, said she tested negative earlier in the day. She will keep her full schedule in Michigan and Georgia on Thursday, though she will be following guidance from the Centers for Disease Control and Prevention on masking and distancing, said Michael LaRosa, her spokesperson.

The president spent much of last week in Israel and Saudi Arabia. White House officials had told reporters that Biden planned to minimize contact during the trip, yet as soon as he exited Air Force One on July 13, he was fist-bumping, handshaking and even was seen in an occasional hug. The CDC says symptoms can appear two to 14 days after exposure to the virus.

Biden has had a minimal public schedule after returning from Saudi Arabia late on Saturday night, attending church the next day and helping to welcome Ukraine's first lady Olena Zelenska to the White House on Tuesday. The president traveled to Massachusetts on Wednesday to promote efforts to combat climate change.

Up to this point, Biden's ability to avoid the virus seemed to defy the odds, even with the testing procedures in place for those expected to be in close contact with him. Prior waves of the virus swept through Washington's political class, infecting Vice President Kamala Harris, Cabinet members, White House staffers and lawmakers. Biden has increasingly stepped up his travel schedule and resumed holding large indoor events where not everyone is tested.

A White House official said Harris tested negative for COVID-19. She was last with the president on Tuesday and spoke with him on the phone Thursday morning. Harris planned to remain masked on the guidance of the White House medical team.

House Speaker Nancy Pelosi said she hoped that Biden's positive test for the virus would cause more Americans to get vaccinated and boosted because "none of us is immune from it, including the president of the United States, and we really have to be careful."

Senate Republican Leader Mitch McConnell on Twitter wished the president "a speedy recovery."

Top White House officials in recent months have been matter-of-fact about the likelihood of the president getting COVID, a measure of how ingrained the virus has become in society — and of its diminished threat for those who are up to date on their vaccinations and with access to treatments.

When administered within five days of symptoms appearing, Paxlovid, produced by drugmaker Pfizer, has been proven to bring about a 90% reduction in hospitalizations and deaths among patients most likely to get severe disease.

Biden is far from the first world leader — and not the first U.S. president — to get the coronavirus, which has infected British Prime Minister Boris Johnson, French President Emmanuel Macron and more than a dozen other leaders and high-ranking officials globally.

When Biden's predecessor, Donald Trump, contracted the disease in October 2020, vaccines were not available and treatment options were limited and less advanced. After being diagnosed at the White House, Trump was given an experimental antibody treatment and steroids after his blood oxygen levels fell dangerously low. He was hospitalized at Walter Reed National Military Medical Center for three days

After more than two years and over a million deaths in the U.S., the virus is still killing an average of 353 people a day here., according to the CDC. The unvaccinated are at far greater risk, more than twice as likely to test positive and nine times as likely to die from the virus as those who have received at least a primary dose of the vaccines, according to the public health agency.

The highly transmissible omicron variant is the dominant strain in the U.S., but scientists say it poses a lower risk for severe illness to those who are up to date on their vaccinations. Omicron’s BA.5 sub-strain, believed to be even more contagious, now makes up more than 65% of U.S. cases.

The coronavirus pandemic helped put Biden in the White House, as he pledged to handle COVID-19 better than his predecessor. After he had initial months of success surging the nation's supply and availability of vaccines, the virus became a worse problem as he struggled to stay ahead of the unpredictable disease.

As cases sharply declined earlier this year, Biden highlighted his administration's efforts to end the pandemic and help the nation regain a sense of normalcy. He said in his March 1 State of the Union address, "It's time for Americans to get back to work and fill our great downtowns again."