Zheng Zhongwei, an official with the National Health Commission and head of China's COVID-19 vaccine development task force, said domestic COVID-19 vaccines have been effective in containing the recent outbreaks due to the contagious Delta variant in areas including Nanjing, Yangzhou and Zhengzhou.

A total of 1,388 locally transmitted cases had been reported as of Wednesday and some of them have received vaccines, he said.

However, more than 95 percent of the severe cases were those that have not been fully vaccinated and none of the severe cases in Yangzhou have been vaccinated, Zheng said.

Based on research of the outbreak in Guangdong province by renowned epidemiologist Zhong Nanshan's team, two doses of vaccines were 70 percent effective against patients with mild symptoms and 100 percent effective against patients with severe symptoms, he said.

COVID-19 booster shots are currently recommended for the elderly or immunocompromised, people planning to travel to foreign regions with surging infections and workers in high-risk sectors, Zheng said.

Whether the wider population should receive an additional dose to boost immunity is still being investigated, he added.