US PRESIDENT Joe Biden’s bout with Covid-19 shows the evolution of the nation’s fight against the pandemic, White House officials said, expressing confidence that the 79-year-old president will avoid the worst of the disease thanks to vaccines and a therapeutic drug largely unavailable before he took office.

Biden’s infection, though, shows the enduring nature of the pandemic he had promised as a presidential candidate to contain, if not defeat. And after killing millions of people worldwide, most of them elderly, the president’s risk of severe illness — while very low, according to medical experts — isn’t zero.

He is experiencing mild symptoms and has begun taking Pfizer Inc’s Paxlovid treatment. He will isolate at the White House while continuing his duties via telephone and Zoom until he tests negative.

Biden worked from the White House residence on Thursday, releasing a video intended to calm fears about his illness. “I’m doing well, getting a lot of work done,” he said, standing on a balcony. “Keep the faith, it’s going to be OK.”

The fallout from Biden’s diagnosis is not yet clear. Vice President Kamala Harris and Chief of Staff Ron Klain have each been deemed close contacts, though Klain told MSNBC on Thursday evening that there have been no cases identified in tracing people Biden spoke with when he may have been contagious.

“I’ve talked to him throughout the day. He’s doing very well, he’s got a sniffly nose, but he is busy at work doing the nation’s business,” Klain said.

Ashish Jha, the White House coronavirus response coordinator, said Biden’s “risk of serious illness is dramatically lower” because he’s fully vaccinated and has received two boosters.

Jha said Biden’s doctor decided he didn’t need treatment beyond Paxlovid and that the president would isolate until he tests negative.

The president returned this week from visiting Saudi Arabia and Israel, and traveled Wednesday to Massachusetts to promote executive actions to address climate change. He was supposed to travel to Pennsylvania on Thursday.

In a letter to staff, Klain said the White House Medical Unit is identifying all of Biden’s close contacts and informing them of his infection.

“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.

Lawmakers who traveled with him to Massachusetts included Senators Elizabeth Warren and Ed Markey as well as Representatives Jake Auchincloss and Bill Keating.

Warren “tested negative today and is following CDC guidance as a close contact,” said her spokeswoman, Alex Sarabia. Matt Corridoni, a spokesman for Auchincloss, tweeted that the congressman tested negative Thursday morning.

Harris tested negative Thursday morning and was last with the president Tuesday, spokeswoman Kirsten Allen tweeted. Harris contracted Covid in April.

First lady Jill Biden, who is visiting Detroit, also tested negative for Covid Thursday morning, she tweeted, adding that she’d spoken to Biden “and he’s feeling fine, just experiencing a few mild symptoms.”

The White House said its contact tracing is focused on alerting people who’ve been exposed, and that they’re not trying to suss out where Biden may have gotten the virus himself.

Representative Bennie Thompson, who chairs the House committee investigating the Jan. 6 insurrection at the Capitol, was unable to preside over a televised hearing Thursday night because he is recovering from the disease. Representatives Gerry Connolly of Virginia and Val Demings of Florida and Senator Tina Smith of Minnesota have all announced infections in the last week.

A White House communications official who traveled with Biden to the Middle East, Meghan Hays, has also contracted Covid-19, according to three people familiar with the matter. She didn’t respond to phone and email messages seeking comment and the White House declined to comment on her infection.

Biden is fully vaccinated and has received two booster shots, though it’s been nearly four months since his most recent dose. Press Secretary Karine Jean-Pierre said the White House will provide a daily update on his condition until he recovers.

The White House has warned that the latest subvariant of the virus — called BA.5 — has an increased ability to evade immunity from prior infections and immunizations. Biden’s test has been sent for sequencing to determine which strain he caught. Those results will be available in less than a week, Jha said.

Earlier this month, the White House said it was stepping up efforts to encourage vaccinations, including booster shots for those eligible.

At the same time, Biden has more freely engaged in social and business activities without precautions such as masks or distancing. In recent weeks, he hosted large groups of lawmakers at the White House for a congressional picnic and an event to mark the passage of legislation intended to address a recent spate of mass shootings. During both events, the president mingled, maskless.

Anyone in a small meeting with the president generally has to take a Covid-19 rapid test. Those who test negative are given a bright-colored bracelet. But the president has recently attended larger events where not all attendees are tested.

Before his trip to the Middle East, White House officials said Biden would be cautious as he met with foreign officials. But almost as soon as he stepped off the plane, Biden began greeting people warmly with handshakes and even embraces.

White House officials have previously said following the infection of top administration officials — including Harris — that it would not be surprising if Biden contracted the virus, even considering expanded safety protocols in place.

Yet Biden aides have increasingly shrugged off the risk, saying privately that they were confident that if he contracted the disease, his case would be mild, thanks to his relative fitness, vaccinations and the availability of treatments.

Biden tested positive first on an antigen test and then again on a PCR test, White House physician Kevin O’Connor said in a memo released by the White House. Biden reported his symptoms to O’Connor after testing positive, Jha said.

Biden’s symptoms include a runny nose, fatigue, and an occasional dry cough, O’Connor said.

O’Connor said that because of Biden’s vaccination status and Paxlovid regimen, he anticipated the president would “respond favorably, as most maximally protected patients do.”

When then-President Donald Trump contracted Covid in October 2020 — before vaccines were available — his treatment was extensive. He received a then-experimental monoclonal antibody cocktail from Regeneron Pharmaceuticals Inc. At Walter Reed National Military Medical Center, where he was hospitalized for three nights, he received both the steroid dexamethasone and Gilead Sciences Inc.’s infused antiviral drug remdesivir.

The five-day course of Paxlovid has become the go-to treatment for keeping high-risk people out of the hospital, despite reports that some people occasionally experience a temporary rebound in symptoms or positive test results. Jha didn’t rule out another round of Paxlovid in the event Biden sees a rebound.

Many other Group of Seven leaders contracted the virus in the past two and a half years of the pandemic. UK Prime Minister Boris Johnson was hospitalised and by his own admission almost died in 2020, when the mortality rates were high, vaccines were unavailable and the disease was much less under control. French President Emmanuel Macron also had the virus in late 2020.

Canadian Prime Minister Justin Trudeau has had Covid twice, including as recently as June, after he attended the Summit of the Americas hosted by Biden.

An estimated 562.7 million people worldwide have contracted Covid-19, resulting in 6.4 million deaths, according to World Health Organization statistics. The virus is still infecting almost 1 million people a day worldwide, without accounting for people who were infected and never tested.

In the US alone, 88.5 million people have gotten sick over the course of the pandemic, according to the WHO. Infections have been on the rise in the US because of more transmissible omicron subvariants like BA.4 and BA.5.

— With assistance from Laura Litvan, Jennifer Jacobs, Courtney Rozen, Jenny Leonard, Erik Wasson and Robert Langreth. — Bloomberg