Mr. Cummings was seen on Friday running out of 10 Downing Street, the prime minister’s official residence, with a backpack, shortly after Mr. Johnson announced that he had tested positive for the coronavirus and would be isolating himself at home.
Mr. Cummings is the latest high-ranking official directly involved in Britain’s outbreak response to test positive for the virus. A critical member of Mr. Johnson’s cabinet, Matt Hancock, the health secretary, also announced last week that he had the illness.
The pathogen has also reached into the royal family. The heir to the throne, Prince Charles, said last week that he had the virus and that he had been experiencing minor symptoms.
Mr. Johnson posted a video on Twitter on Sunday urging Britons to stay at home, appearing in a suit and tie but with a noticeably hoarse voice. He thanked those health care workers who were coming back into the National Health Service, or N.H.S., “in such huge numbers.” Some 20,000 former staff members are returning to the health system to help in the coronavirus response.
Dr. Jenny Harries, the British deputy chief medical officer, said it could be six months or more before a return to normal, with lockdowns being reassessed every three weeks. She said that if the strategy was successful, the country could effectively limit the peak of cases in the short term, but that measures would have to continue.
“We must not then revert to our regular way of living, that would be quite dangerous,” she said during a Sunday evening news conference. “If we stop then, all of our efforts will be wasted and we could potentially see a second peak.”
Thousands of airline staff who were grounded as travel came to a grinding halt amid worldwide restrictions will also be joining the efforts, according to the N.H.S. Cabin crews from Virgin Atlantic and easyJet have been asked to work at coronavirus field hospitals across the country as part of the health service’s response, the health service said in a statement.