President Joe Biden has tested positive for Covid-19 and is experiencing "mild symptoms," White House press secretary Karine Jean-Pierre said Thursday.

The mild symptoms and diagnosis protocol for Biden — a double-boosted 79-year-old at high risk for experiencing severe illness — so far will mean “working and resting” at the White House for the rest of the day, according to a senior administration official. This is the first time Biden has tested positive for Covid-19, and he last tested negative on Tuesday, per Jean-Pierre. Vice President Kamala Harris and first lady Jill Biden have tested negative on Thursday.

“This morning, President Biden tested positive for Covid-19. He is fully vaccinated and twice boosted and experiencing very mild symptoms. He has begun taking Paxlovid,” she said, referring to Pfizer's antiviral drug, which is available via emergency use authorization from the US Food and Drug Administration for treatment of mild to moderate Covid-19 in people 12 and older who are at high risk of severe illness. It requires a doctor's prescription.

“Consistent with CDC guidelines, he will isolate at the White House and will continue to carry out all of his duties fully during that time,” she added.

Harris is expected to keep her normal schedule. The senior administration official said there are no plans and no need to transfer executive powers to Harris — as was done last November for 85 minutes, when Biden was under anesthesia for a routine colonoscopy.

Biden did not have a fever Thursday morning, the White House's coronavirus coordinator Dr. Ashish Jha told CNN. And Dr. Kevin O'Connor, the President's physician, wrote in a letter about the President's condition that Biden was “experiencing mild symptoms, mostly rhinorreha (or ‘runny nose’) and fatigue, with an occasional dry cough, which started yesterday evening.”

Biden tested positive for Covid-19 on an antigen test Thursday morning, part of routine screening, and later was confirmed by a PCR test, his physician said.

O'Connor noted that the President met the FDA's criteria for use of the antiviral Paxlovid, adding, “I anticipate that he will respond favorably, as most maximally protected patients do.”

“I talked to him just a few minutes ago. He's doing fine, he's feeling good,” first lady Jill Biden told reporters in Detroit.

The first lady, her spokesman Michael LaRosa told CNN, will keep her schedule on Thursday, which includes a stop in Georgia before heading to WIlmington, Delaware. She is also double boosted, LaRosa said.

Biden received his first two doses of the Pfizer/BioNTech Covid-19 vaccine ahead of his inauguration in January 2021, his first booster shot in September and his second booster vaccination on March 30.

Due to his age, Biden is at an increased risk for a more severe case of Covid-19, although the US Centers for Disease Control and Prevention says older adults being fully vaccinated and boosted significantly reduces their risk of hospitalization and death.

Jean-Pierre and Jha are expected to brief the press on the President's diagnosis later Thursday afternoon.