President Biden tests positive for COVID-19

President Joe Biden tested positive for COVID-19 for the first time Thursday morning, the White House said.

Biden, 79, has "very mild symptoms" and is taking Paxlovid, White House press secretary Karine Jean-Pierre said in a statement.

Biden is experiencing a runny nose, dry cough and fatigue, White House COVID-19 Response Coordinator Dr. Ashish Jha said.

The president felt "totally normal" during the day Wednesday, Jha said. Biden’s symptoms started Wednesday evening, according to the physician to the president, Dr. Kevin O’Connor, and he then tested positive in the morning as a part of routine testing, Jha said.

Biden tweeted that he's "doing great" and "keeping busy!"

Jean-Pierre said an update will be provided every day as Biden "continues to carry out the full duties of the office while in isolation" at the White House.

Close contacts, "including any Members of Congress and any members of the press who interacted with the President during yesterday’s travel," will be informed on Thursday, Jean-Pierre said.

Biden traveled to Somerset, Massachusetts, on Wednesday where he announced executive actions to address climate change. Biden delivered his speech outside and met with local officials.

Sen. Elizabeth Warren, D-Mass., who was with Biden at his event Wednesday, tested negative on Thursday, a spokesperson said.

The president greeted Ukrainian first lady Olena Zelenska at the White House on Tuesday.

Vice President Kamala Harris, who was with the president on Tuesday, tested negative Thursday morning. Her office said she'll remain masked and continue her schedule as planned.

Zelenska said she doesn't have any symptoms. She said a negative test was required ahead of Tuesday's meeting, adding that she plans on re-testing.

The president will work in isolation until he tests negative, Jean-Pierre said.

"He has been in contact with members of the White House staff by phone this morning, and will participate in his planned meetings at the White House this morning via phone and Zoom from the residence," Jean-Pierre said.

Biden is fully vaccinated and received two boosters; his second booster shot was March 30.

Biden was last tested for COVID-19 on Tuesday, when he tested negative, she added.

White House chief of staff Ron Klain said in an email to staff, "We have said for some time that there was a substantial possibility that the president -- like anyone else -- could get COVID, and we have prepared for this possibility."

First lady Jill Biden tested negative Thursday morning in Detroit and will keep her full schedule in Michigan and Georgia through the day, her office said. She will continue following CDC guidance with masking and distancing, her office said.

Jill Biden said she spoke with the president Thursday morning and said "he’s doing fine" and "feeling good."

Sen. Bob Casey, D-Pa., tweeted that he spoke with Biden on the phone and that he "sounded great and is in good spirits."

Because Biden is vaccinated and double boosted, his "risk of serious illness is dramatically lower," Jha said at a Thursday press briefing. "He is also getting treated with a very powerful antiviral that further reduces his risk of serious illness."

The virus has been sent for sequencing to determine the COVID-19 variant the president has, Jha said.

ABC News Chief Medical Correspondent Dr. Jennifer Ashton said the biggest factor in treatment will be the president's age.

"That is why," she said, it's "no surprise that he's being treated with the antiviral pill Paxlovid. It's been shown in clinical trials to be 89% effective in reducing the risk of severe COVID-19 illness, meaning hospitalization or death."

Ashton stressed, "He is going to be closely monitored at the White House by the personal physician of the president, and if anything looks like it is going in the wrong direction, I absolutely expect that he would be hospitalized, if nothing else than for more close observation. But remember, the White House is not like your home or my home -- they can do a lot of medical monitoring and observation and testing right there."

Former President Donald Trump was briefly hospitalized at Walter Reed Medical Center in October 2020 after he tested positive for COVID-19.

Paxlovid, an antiviral pill developed by Pfizer, was approved by the Food and Drug Administration for those aged 12 and older in December 2021.

Patients take three pills twice daily over the course of five days. The pill was hailed as a game-changer because it was the first COVID-19 treatment that did not require an infusion, making it more convenient to give to patients.

Paxlovid is made up of two medications: ritonavir, commonly used to treat HIV and AIDS, and nirmatrelvir, an antiviral that Pfizer developed to boost the strength of the first drug. Together, they prevent an enzyme the virus uses to make copies of itself inside human cells and spread throughout the body.

Although there have been increasing reports of a rebounding phenomenon after taking Paxlovid, Ashton said the benefits still outweigh any of the risks.

"What would be the worst-case scenario if President Biden took Paxlovid and got this so-called rebound phenomenon? Just more time being in isolation. The benefits are clear based on the clinical trials of what this drug can do in terms of reducing the risk of hospitalization and death, and that's why it is a straightforward and easy clinical decision to make to give him that medication," she said.

The vice president tested positive for COVID-19 in April. Dr. Anthony Fauci, a senior adviser to the president on the pandemic, tested positive last month.

ABC News' Mary Kekatos, Molly Nagle and Trish Turner contributed to this report.