Qingdao, Shandong province, is on high alert against the latest outbreak of COVID-19 cases involving middle school students after its county-level city Laixi reported a total of 91 cases by the end of Saturday.

The outbreak happened Friday night when the local health authority confirmed the infection of three students at the Laixi No 7 Middle School, followed by a mass nucleic acid testing in Laixi, about 100 kilometers away from Qingdao's downtown area, which resulted in the discovery of other 88 cases.

The cases include 77 students and nine teachers from Laixi No 7 Middle School, one student from another school in Laixi and four contacts, according to a news conference held by the information office of Qingdao municipal government on Sunday morning.

One of the teachers doing volunteer teaching job in Laixi and traveling between Laixi and Qingdao’s urban district of Shinan was found infected in the district.

"All the cases are mild symptoms and their condition is currently stable," said Xue Qingguo, vice-mayor of Qingdao, at the conference.

According to gene sequencing, all the cases are Omicron but currently have no homology with the virus genome of local or imported cases in other parts of the country.

After the virus outbreak, Laixi government took a series of actions to stop the virus spread, including limiting access to and departure from Laixi.

The Laixi No 7 Middle School has been listed as the high-risk area.

As the outbreak mainly happened at a school campus, where a number of teachers and students are easily exposed to virus, officials and experts expects that more new cases would be discovered but the city has abundant medical resources to handle the virus outbreak.

Xue said 14 medical teams of about 550 people from eight hospitals in Qingdao are on call and two hospitals with more than 1,500 wards as backup.

A second round of mass nucleic acid testing in Laixi will be organized soon, Xue added.

Besides, Qingdao's Huangdao district also reported six cases from Tuesday to Friday. The first infected person is a bus driver and the rest are his family members and relatives.

The local authority said on Sunday morning that the epidemic was accidently caused by a parcel delivered from another province and has no relation with Laixi cases.