The number of serious COVID-19 infections has risen to nine in Nanjing, East China's Jiangsu province, as of Saturday morning, accounting for about 4 percent of total confirmed cases in the city that has been battling a local outbreak since July 20, a senior health official said on Saturday.

Guo Yanhong, an official with the National Health Commission's Bureau of Medical Administration, said all serious patients are infected with the Delta variant, a highly transmissible strain, and they are now in stable condition.

These patients did not exhibit typical symptoms at the beginning but their condition quickly worsened. "It took an average of five days for them to be become serious cases," she said, adding that most of them suffered chronic illnesses before, such as diabetes and high blood pressure.

She added that severe cases are now receiving treatment at designated hospitals and the commission has dispatched medical experts there.

She said therapies delivered to them are positive and effective.