Nancy Pelosi and other lawmakers test positive for the coronavirus.

WASHINGTON — Speaker Nancy Pelosi of California tested positive for the coronavirus, her office said on Thursday, the latest high-profile official to do so as the virus spreads through Washington.

Drew Hammill, a spokesman for Ms. Pelosi, said she was asymptomatic, and had previously been vaccinated and boosted. He added that Ms. Pelosi, 82, had tested negative earlier this week.

Ms. Pelosi, who is second in the line of succession to the presidency after Vice President Kamala Harris, was the highest-ranking in a string of positive coronavirus tests among top officials in Washington, underscoring that the pandemic is not over, even as restrictions loosen.

Lawmakers and their staffs on Capitol Hill were informed on March 30 that they were now allowed to receive a second booster dose if they met criteria, after federal health officials cleared them for many people the day before. Ms. Pelosi received her second booster last month, according to her office, and it takes about a week for the immune system to ramp up. It is difficult to know exactly how someone was exposed to and infected with the virus.

“The speaker is fully vaccinated and boosted, and is thankful for the robust protection the vaccine has provided,” Mr. Hammill said, adding that Ms. Pelosi would isolate consistent with guidance from the Centers for Disease Control and Prevention.

The speaker “encourages everyone to get vaccinated, boosted and test regularly,” he said.

Ms. Pelosi was among the lawmakers who appeared maskless with President Biden at a bill-signing ceremony at the White House on Wednesday. On Thursday, White House officials said Mr. Biden tested negative Wednesday evening, and despite interactions in recent days and Ms. Pelosi’s presence at White House events, she was not considered a close contact of the president, defined by the C.D.C. as someone less than six feet away for more than 15 minutes over a 24-hour period. Mr. Biden would continue to be tested regularly, White House officials said.

At a news briefing on Thursday, Jen Psaki, Mr. Biden’s press secretary, said there were “incredibly stringent protocols” at the White House that went beyond C.D.C. guidelines.

“Every member of the staff is on a regular testing protocol,” she said. “If you’re going to see him in person, whether you were traveling with him, or you’re meeting in the Oval Office, you will be tested.”

Ms. Pelosi’s positive test came just before her weekly news conference, which was subsequently canceled. Mr. Hammill said a planned congressional delegation to Asia, which Ms. Pelosi had been scheduled to lead, would be postponed.

Positive tests have been reported among top officials and journalists who attended other recent parties and events, including the Gridiron Club dinner, an annual black-tie roast between journalists and presidential administrations held over the weekend.

Just hours after becoming one of three Republicans to support the confirmation of Judge Ketanji Brown Jackson to the Supreme Court, Senator Susan Collins of Maine tested positive for the virus. Ms. Collins’s office said the senator, who wore a mask in the Senate chamber while voting on Thursday, was experiencing mild symptoms and would isolate herself.

Two Biden administration officials, Commerce Secretary Gina Raimondo and Attorney General Merrick B. Garland, said on Wednesday that they had tested positive for the virus. Mr. Biden, along with Ms. Pelosi, did not attend the Gridiron, but appeared mask-free at multiple events this week.

On Thursday, the virus continued to tear through Capitol Hill: Senator Raphael Warnock of Georgia and Representative Angie Craig of Minnesota, both Democrats, said on Twitter that they had tested positive. That followed two more House Democrats, Representatives Derek Kilmer of Washington and Gregory W. Meeks of New York, who said Wednesday on Twitter that they had tested positive and were asymptomatic.

Another Washington, D.C., politician, Muriel Bowser, the mayor, also said on Twitter on Thursday that she tested positive a day earlier and was experiencing mild symptoms.

Earlier this week, Representatives Joaquin Castro of Texas and Adam B. Schiff of California, both Democrats, said they had tested positive after attending the Gridiron. Other Democrats who did not attend the dinner have announced positive tests, including Representative Katherine M. Clark of Massachusetts, the assistant House speaker, and Representative Scott Peters of California.

Ms. Pelosi’s infection has had ripple effects on official Washington business even among people who have been exposed to her but not tested positive. Xavier Becerra, the health and human services secretary, appeared at a news conference on Thursday wearing a mask for most of the event, saying that he had been around Ms. Pelosi days earlier. The briefing, held to mark World Health Day, included Dr. Tedros Adhanom Ghebreyesus, the director general of the World Health Organization, who had visited Ms. Pelosi this week and also wore a mask Thursday.

As of Wednesday, an average of 144 new virus cases per day were reported in Washington, D.C., a 135 percent increase over the past two weeks, according to a New York Times database.

Reporting was contributed by Katie Rogers, Adeel Hassan, Alyssa Lukpat and Noah Weiland.