(CNN) — President Joe Biden tested positive for Covid-19 on Wednesday, disrupting a key campaign event meant to shore up support with Latino voters at a critical juncture in the election.

Biden, 81, was experiencing mild symptoms and has received his first dose of the antiviral drug Paxlovid, according to his doctor.

“I feel good,” Biden, who is fully vaccinated and boosted, told reporters Wednesday in Las Vegas, flashing a thumbs-up before boarding Air Force One to head to his Delaware residence. The president will self-isolate there in line with US Centers for Disease Control and Prevention guidance.

Biden and his aides have repeatedly cited recent stops on the campaign trail as a demonstration of his vigor as the presumptive Democratic nominee, an attempt to tamp down growing anxieties within the party. Wednesday’s event in Las Vegas – where he was expected to speak at the UnidosUS annual conference – was billed as an example of that direct engagement, in this case with a diverse coalition.

The UnidosUS event, however, came to a screeching halt with the news that the president had tested positive for Covid-19 following an active day participating in an interview and meeting voters at a local supermarket.

“I was just on the phone with President Biden. And he shared his deep disappointment at not being able to join us this afternoon. The president has been at many events as we all know and he just tested positive for Covid,” UnidosUS President and CEO Janet Murguía told attendees, to audible gasps, after Biden was an hour and a half late to his scheduled appearance onstage.

It’s unclear where and when the president contracted Covid-19. But the sudden diagnosis will place him behind closed doors at a time when Democratic lawmakers have been clamoring to see him out front.

The CDC recommends staying home until the infected person has been without a fever – with no fever-reducing medicine – for at least 24 hours and symptoms have been improving for 24 hours. It then recommends wearing a mask for five days after.

As for Biden’s schedule in the coming days and his ability to stay engaged, a White House official told CNN that the president has had Covid-19 before and gotten through those periods by doing a lot of video calls. Something similar should be expected in the days ahead, the official said.

A campaign source separately told CNN that the Biden campaign will readjust and do as much as possible remotely.

Biden was at his Rehoboth Beach, Delaware, residence last weekend as well, making calls to key groups of lawmakers, some of whom continue to harbor deep concerns about his viability as a candidate.

The attempted assassination of former President Donald Trump appeared to put public calls for Biden to step aside as the party’s presumptive nominee on pause. But they returned as Biden hit the campaign trail again this week.

Just hours before the White House announced Biden’s Covid-19 diagnosis, California Rep. Adam Schiff, a Senate candidate and former House Intelligence chairman, became the most prominent elected Democrat to publicly call on the president to drop out of the race.

As Trump is set to formally accept the GOP nomination for president at this week’s Republican National Convention in Milwaukee, Democratic fears persist that Biden cannot defeat him and risks taking down Democrats in down-ballot races with him in November.

But even though anger and panic has been steadily rising inside the Democratic Party after Biden’s alarming debate performance in Atlanta nearly three weeks ago, the White House and the Biden campaign are in a new place, multiple Democratic officials told CNN.

“The private conversations with the Hill are continuing,” a senior Democratic adviser tells CNN, speaking on condition of anonymity to avoid alienating the campaign and the White House. “He’s being receptive. Not as defiant as he is publicly.”

“He’s gone from saying, ‘Kamala can’t win,’ to ‘Do you think Kamala can win?” the adviser said. “It’s still unclear where he’s going to land but seems to be listening.

Despite Biden’s diagnosis, Vice President Kamala Harris was set to keep her Thursday appearance at a campaign event in Fayetteville, North Carolina, as planned, an administration staffer said Wednesday evening. Harris’ husband, second gentleman Doug Emhoff, tested positive for Covid-19 earlier this month – though the vice president tested negative for the virus at the time.

Biden, who was spotted not wearing a mask Wednesday before boarding Air Force One, was wearing one in the car and wore one aboard the aircraft to Delaware, a White House official said.

Biden had “upper respiratory symptoms, to include rhinorhea (runny nose) and non-productive cough, with general mailaise,” a note from his doctor provided by White House press secretary Karine Jean-Pierre said.

“He felt okay for his first event of the day, but given that he was not feeling better, point of care testing for COVID-19 was conducted, and the results were positive for the COVID-19 virus,” the note continued.

The president first tested positive for Covid-19 in July 2022 and suffered a second, so-called rebound case in the days after.

Biden has received multiple Covid-19 vaccine booster shots, most recently in September 2023, according to a memo from his physician. During his first bout with the disease, he experienced mild symptoms, including runny nose, fatigue, high temperature and a cough and was treated with Paxlovid.

Covid-19 levels across the country have risen in recent weeks, according to the CDC. The most recent data shows that during the week ending July 6, there was a 23.5% increase in emergency visits for Covid-19 compared with the previous week. The CDC also reports that the viral activity level for Covid-19 in wastewater is high nationally as of July 6.

This story has been updated with additional developments.

CNN’s Dana Bash, MJ Lee, Eva McKend, Sam Fossum, Katia Hetter and Megan Trimble contributed to this report.