The government’s decision to modify the protocol, unusual in its swiftness, underscored the pressure it is under, particularly in Hubei’s capital, Wuhan, a city of 11 million where pleas from residents desperate for medical help have caused widespread public anger. The vast majority of deaths in China from the coronavirus have been recorded in Wuhan.

Dr. Joe Chang, a specialist at the department of radiation oncology at MD Anderson Cancer Center in Houston, said the use of CT scans to screen patients in Hubei made sense.

“The issue now is the number of patients,” he said. “No country can be prepared for these kinds of numbers.”

In Wuhan, long lines of people snake around the city’s hospitals, and many residents with fevers, coughs and other symptoms of the virus say they can’t get tested.

Yuan Xiuhua, a 49-year-old Wuhan resident, came down with a fever on Jan. 22 and went to a hospital, where CT scans showed lesions in her lungs. She has repeatedly asked her community district, which prioritizes who gets tested for the coronavirus, to give her one, but was told that because there were too many possible cases, she was better off isolating herself at home. Her husband, she said, recently came down with a fever and diarrhea.

Ms. Yuan said she was still calling her community district every day to ask for a test.

“They keep on saying that there are no free spots,” she said. “They didn’t provide me with any help. They’ve just made me wait.”

Faced with criticism over the slow response in the first weeks of the outbreak, Beijing has ordered increasingly extreme measures. The government in Wuhan was to hospitalize or place in mass quarantine centers all confirmed infected patients.