Nanjing, capital of east China's Jiangsu Province, has seen a resurgence of locally transmitted COVID-19 cases since a number of airport workers tested positive for the coronavirus on July 20.

Though many of the infected have been vaccinated, Shao Yiming, a researcher at Chinese Center for Disease Control and Prevention said it's not a surprise.

"The protective role of vaccines is divided into three levels: The first level is to prevent infections, the second level is to prevent symptoms and reduce the severity of the disease and avoid death, and the third level is to prevent the virus from spreading to others. The efficacy rates we now see of 90, 80 or 70 percent refer to the second level of protection, not the first. So the infections we're now seeing are expected," Shao said.

"This raises challenges for the next stage of prevention and control efforts, both through public health measures and vaccinations. Scientific research is being sped up. We'll gradually see fewer and fewer such occasions," he added.

Local authorities said the more infectious Delta variant is spreading in Nanjing. At least 100 nations have reported infections caused by the variant.

According to a report issued by the Boao Forum of Asia on Thursday, more than 70 percent of the new global cases are related to the Delta variant. Shao Yiming said experts are considering a booster dose to combat variants like this.

"Regarding mutations, we can use a booster dose to enhance the basic immunity that vaccinated people have gained," said Shao.

Shao said the infections in Nanjing show China is facing pressure, as the virus comes in from abroad. He suggested special measures be taken for members of high-risk groups, especially those working at entry ports.