As the Delta variant of the coronavirus spreads in southeastern China, doctors say they are finding that the symptoms are different and more dangerous than those they saw when the initial version of the virus started spreading in late 2019 in the central city of Wuhan.

Up to 12 percent of patients become severely or critically ill within three to four days of the onset of symptoms, said Guan Xiangdong, director of critical care medicine at Sun Yat-sen University in the city of Guangzhou, where the outbreak has been centered.

The testimonies from China are the latest indication of the dangers posed by Delta, which the World Health Organization last month labeled a “variant of concern.” First identified this spring in India, where it was blamed for widespread suffering and death, Delta has since become the dominant variant in Britain, where doctors suggest that it is more contagious and may infect some people who have received only one of two doses of a Covid-19 vaccine.

The authorities said that they had conducted 32 million tests in Guangzhou, which has 18 million people, and 10 million in the adjacent city of Foshan, which has seven million.