Qingdao, a coastal city in Shandong province, is on high alert due to a COVID-19 outbreak involving middle school students after Laixi, a county-level city under its jurisdiction, reported 118 cases by Sunday afternoon.

The outbreak was detected on Friday night, when the local health authority confirmed three students at Laixi No 7 Middle School were infected. Subsequent mass nucleic acid testing in Laixi, about 100 kilometers from downtown Qingdao, discovered another 115 cases.

The cases comprise 100 students and nine teachers from the middle school, two students from another school in Laixi and seven contacts, according to a news conference held by the Qingdao city government's information office on Sunday afternoon.

One volunteer teacher in Laixi who traveled between the city and Qingdao's urban Shinan district was found to be infected.

"All the cases have mild symptoms, and their conditions are currently stable," Xue Qingguo, vice-mayor of Qingdao, told a news conference on Sunday morning.

According to gene sequencing, all the cases are infected with the Omicron variant of the novel coronavirus but not linked to the virus genome of local or imported cases in other parts of the country.

After the COVID-19 outbreak, the Laixi government took a series of actions to stop the spread of the virus, including limiting access to and departures from Laixi. Several areas in the city have been put under control, with Laixi No 7 Middle School listed as the high-risk area.

A total of 5,292 people within the city have been isolated, and 119 people who had already left the city had been traced by 6 am on Sunday, the morning news conference was told.

As the outbreak mainly happened at a day school campus, where a number of teachers and students could have been easily exposed to the virus and possibly caused transmission at the community level, officials and experts expect more new cases will be discovered. They said the city has abundant medical resources to handle the outbreak.

Xue said 14 medical teams comprising about 550 people from eight hospitals in Qingdao are on call, and two hospitals with more than 1,500 isolation ward beds can provide backup.

A second round of mass nucleic acid testing will start in Laixi on Monday morning, Xue added.

Laixi has made efforts to ensure that disruptions to people's normal lives are kept to a minimum.

Laixi Vice-Mayor Sun Shoubin said five supermarkets in the city have enough supplies, with 32.5 metric tons of rice and 36.5 tons of flour in stock.

"The price of daily necessities such as eggs and meat is also stable," he added.

Separately, Qingdao's Huangdao district reported six confirmed COVID-19 cases from Tuesday to Friday. The first infected person was a bus driver, and the rest were his family members and other relatives.

Qingdao authorities said on Sunday afternoon that the outbreak was accidentally caused by contact with a virus-tainted object and was not related to the cases in Laixi, without giving more details.