President Biden has tested positive for the coronavirus, starkly illustrating the reach of a pandemic that has killed more than 1 million Americans and left few untouched, though he is experiencing “very mild symptoms,” the White House said Thursday.

Biden, who at 79 is in a relatively high-risk group, experienced a runny nose, fatigue and a dry cough beginning Wednesday evening, according to White House physician Kevin O’Connor. Biden immediately canceled a planned trip to Pennsylvania on Thursday and will isolate at the White House for at least five days.

The president “will continue to carry out all of his duties fully during that time,” said White House press secretary Karine Jean-Pierre, adding that the president has begun taking Paxlovid, an antiviral therapy that has proved highly effective in mitigating coronavirus symptoms.

O’Connor said in a letter disclosing the test result that he expects Biden to respond well to Paxlovid “as most maximally protected patients do.” Biden is fully vaccinated for coronavirus and has received two booster shots.

White House coronavirus response coordinator Ashish Jha said Biden’s risk of serious illness was “dramatically lowered” because he has received two initial vaccine doses and two booster shots, and that his oxygen levels were normal. Biden will return to normal activities once he tests negative, Jha said.

Many of Biden’s associates had expected the president to get covid-19 long before now, given the number of people he meets and his tendency to give hugs and handclasps. Biden has a habit of grasping people by the arm or otherwise physically engaging those he encounters.

His diagnosis punctuates the trajectory of a pandemic that has ripped through the United States and the globe, ensnaring everyone from world leaders to senators to pop stars to millions of Americans going about their daily lives.

In a video released on Twitter Thursday, Biden — dressed in a navy blue suit and standing on a White House balcony — assured the public that he had only mild symptoms and was continuing to work.

“I’m doing well, getting a lot of work done, going to continue to get it done. And in the meantime, thanks for your concern and keep the faith,” Biden said. “It’s going to be okay.”

The pandemic has bitterly divided the country. Biden won the presidency in no small part because he positioned himself as someone who would take the pandemic seriously, in contrast with former president Donald Trump, who also contracted the disease and long dismissed covid-19 as a minor irritant.

As president, Biden has gone to great lengths to protect himself against the disease, but the infection comes as the highly contagious omicron subvariant BA.5 rips across the country and as the virus has increasingly seeped into his inner circle, infecting everyone from his family members to many of his top advisers, including chief medical adviser Anthony S. Fauci.

BA.5 has quickly become dominant in the United States and has shown a remarkable ability to escape the immunity conferred by vaccinations and previous coronavirus infections. While reported coronavirus cases for the United States hover at a little more than 100,000 per day, experts say that wildly underestimates the true number, given that many people test at home or do not test at all, and have said cases could be as many as 1 million per day. Hospitalizations have held relatively steady despite increasing cases.

White House officials said they expect to know in less than a week whether the president himself is infected with the BA.5 variant. Like most patients, the president is going off other medications while on Paxlovid — in Biden’s case, a cholesterol drug and a blood thinner — and will resume when his Paxlovid course ends in five days.

Biden’s infection underscores the vexing challenge facing the administration in trying to control the pandemic. Most Americans have long done away with mask-wearing, social distancing and other virus mitigation measures.

Meanwhile, increasingly contagious subvariants have found new ways to infect, and even reinfect, those who are vaccinated and boosted, and roughly 70 percent of Americans have now contracted the coronavirus at some point. The White House has said relatively little about the current surge in infections, aware that Americans have little patience left for measures that restrict their ability to go about their day-to-day lives.

On Thursday, White House officials sought to use the moment to reiterate their message on the importance of getting vaccinated and urged those over 50 who have not gotten a booster shot this year to do so.

“It’s a reminder of the reason that we all work so hard to make sure every American has the same level of protection the president has, that every American has the same level of immunity the president has,” Jha said.

Despite Biden’s age, several experts and a person familiar with his medical condition said they were optimistic his infection would remain mild. Helen Boucher, interim dean of Tufts University School of Medicine and an infectious-disease physician, said Biden is “as well protected as he could be,” adding she was encouraged that he started Paxlovid early.

Boucher said Biden’s physicians are likely to closely monitor whether the president develops shortness of breath, a worsening of symptoms, an inability to eat or drink, or other signs that he’s getting sicker. But she said studies have shown that boosters offer significant protection against severe illness and death.

“While I’d be happier if he didn’t have covid, I am relatively reassured by the fact that he’s fully boosted and has mild symptoms and has begun taking the Paxlovid,” Boucher said.

Paxlovid has proved highly effective in keeping people out of the hospital, especially older patients who are at increased risk. There have been anecdotal cases of a “bounce back” after taking Paxlovid, in which people recover after taking the antiviral but then test positive again a couple days later. But those recurrences are rarely, if ever, severe.

In a note to White House staffers Thursday, White House Chief of Staff Ron Klain said he had spoken to the president several times Thursday morning and that Biden was continuing to work via phone and Zoom.

“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,” Klain wrote in the note, which was first reported by CNN and confirmed by The Washington Post. “We are now executing on our plan so that the President can continue to work seamlessly from the Residence.”

The White House has strict protocols in place to help protect the septuagenarian president, often including masking and staying at least six feet away from Biden when indoors. Biden has also held meetings via Zoom, worn a mask during close encounters and conducted many gatherings in a socially distanced manner.

However, the famously tactile president has not always been able to shake his glad-handing political style of a half-century. White House officials said ahead of a trip to the Middle East last week that he would reduce contact and increase masking given the recent surge in cases, but Biden quickly eschewed those measures in Israel, opting to shake hands and backslap with politicians and residents of a country he has often visited.

Beyond that, Biden has traveled extensively in recent weeks, including to Somerset, Mass., on Wednesday with a contingent of staffers and members of Congress, including Sens. Elizabeth Warren (D-Mass.) and Edward J. Markey (D-Mass.).

The White House said it would inform all those deemed to have had close contact with the president within the past 48 hours, which the Centers for Disease Control and Prevention defines as being less than six feet away from someone with covid-19 for at least 15 minutes. Jean-Pierre said she did not know the total number of individuals that includes, and it is not clear how the president got infected.

The White House has said Biden undergoes regular coronavirus tests, and the president tested negative for the virus Tuesday.

Biden’s plans to travel to Pennsylvania on Thursday and Florida on Monday were quickly suspended, but the president was at pains to show Americans he was not seriously ill.

“Folks, I’m doing great,” he tweeted. “Thanks for your concern. Just called Senator Casey, Congressman Cartwright, and Mayor Cognetti (and my Scranton cousins!) to send my regrets for missing our event today.”

He was referring to Sen. Robert P. Casey Jr. (D-Pa.), Rep. Matthew Cartwright (D-Pa.) and Scranton Mayor Paige Cognetti, all of whom were scheduled to be at Biden’s events in Pennsylvania.

Casey tweeted, “I just hung up with @POTUS who sounded great and is in good spirits. Looking forward to welcoming him back to [northeastern Pennsylvania] sometime soon. Get vaccinated and boosted, everyone.”

First lady Jill Biden said she tested negative for the coronavirus Thursday morning. She proceeded with a planned trip to Detroit and said she would follow CDC guidance and wear a mask.

Biden has stood out for his precautions against covid-19 as early as his presidential campaign, when he wore a mask and limited his travel at a time when few public figures were doing so, occasionally earning Trump’s ridicule.

That approach protected the president as each new Biden aide or relative tested positive, including Vice President Harris, Secretary of State Antony Blinken, Biden’s sister Valerie Biden Owens and his daughter Ashley, to name a few. After each infection, those aides and family members were not deemed close contacts with the president, who managed to avoid significant exposure.

But the precautions were relaxed somewhat as the pandemic has begun to recede. The White House, meanwhile, adjusted its public messaging to urge Americans to resume their normal lives, even while urging them to continue taking reasonable precautions, especially getting vaccinated.

Covid-19 has now affected two presidents. Just a month before the 2020 presidential election, Trump tested positive for the coronavirus, a particularly alarming development because at the time, vaccines and treatments such as Paxlovid were not available.

Trump, despite his publicly downplaying his symptoms, was rushed to Walter Reed National Military Medical Center on Marine One as his blood oxygen levels plunged, and White House officials feared that he might need to be placed on a ventilator.

Although Trump ultimately recovered, his bout with covid helped deepen the perception that he and his team had mismanaged the virus and behaved recklessly. After his diagnosis, he rode in a closed limousine with Secret Service agents, and when he returned to the White House he made a public gesture of removing his mask on the building’s balcony.

After Trump’s presidency, a book by his former chief of staff, Mark Meadows, revealed that Trump had received a positive coronavirus test Sept. 26, about a week before the public announcement that he had covid-19 and his subsequent hospitalization.

Trump and Meadows hid that test from the public, and there is no way to know whether it was a false positive or an early indicator of the virus that ultimately led to his hospitalization. But according to a Post analysis, Trump came in contact with more than 500 people during that period.

There are no plans to move Biden from the White House to Walter Reed, given the mildness of his symptoms, though that could always change.

“Walter Reed is always on standby for presidents,” Jha said, speaking at the White House. “That’s true whether the president has had covid or not. … He’s getting the care he needs here.”