Joe Biden, the US president, has tested positive for COVID.
The president, 79, is “experiencing very mild symptoms”, his press secretary said as she announced the news.
Karine Jean-Pierre said Mr Biden tested positive on Thursday morning. He is fully vaccinated and twice boosted.
In a video message, the president thanked people for their concern, saying he was “doing great” and “getting a lot of work done”.
Dr Ashish Jha, the White House coronavirus response coordinator, said Mr Biden’s symptoms began on Wednesday and include a runny nose, dry cough and fatigue. He said the president went to bed and felt fine, but did not sleep well.
The White House said he has begun taking an antiviral treatment, Paxlovid, and will isolate while continuing to carry out duties “fully”.
Jill Biden, the first lady, confirmed she has tested negative for the virus and had spoken to the president. “He’s feeling fine, just experiencing a few mild symptoms,” she said.
The prime minister’s spokesman said Boris Johnson sends Mr Biden his “best wishes” and hopes he has a speedy recovery.
The president, who is the oldest person to serve as US president, was out on Wednesday making a speech on climate change at a former coal plant in Massachusetts.
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Mr Biden was in contact with members of staff by phone on Thursday morning, the White House said, and would participate in his planned meetings via phone and Zoom, resuming in-person work when he tests negative.
“The president’s last previous test for COVID was Tuesday, when he had a negative test result,” it said.
On 16 June, NBC News reported that the president tested negative for the virus twice during that week.
The disclosure came after Ms Jean-Pierre repeatedly refused to reveal the president’s testing status, a reversal from previous practice.
Multiple members of the Biden administration and other senior figures in Washington have tested positive for the virus in recent months, including vice-president Kamala Harris in April, House of Representatives speaker Nancy Pelosi and Attorney General Merrick Garland.
All have since tested negative and resumed working.