Imported cases of the Omicron variant were reported in East China's Zhejiang province as early as one week ago, with sufferers put under tight quarantine in a closed-loop system, according to a press conference Tuesday afternoon.

All of these patients were infected in their upper respiratory tracts, with mild symptoms such as dry cough and sore throat, said Sheng Jifang, director of the Department of Infectious Diseases at the First Affiliated Hospital of the College of Medicine at Zhejiang University in Hangzhou.

Vaccination may be the main reason the lungs of these patients have not been infected, as antibodies against the virus and strong cell immunity had been generated.

Sheng added vaccination has proven to produce the effective antibodies, with no lung infection found.

She also reiterated China's vaccines are inactivated and complete, producing antibodies and powerful cellular immunity.

Fang Xiaoying in Hangzhou contributed to this story.