Two major COVID-19 transmission chains have been identified in Shaoguan, Guangdong province, which recently reported multiple sporadic cases, a senior disease control and prevention official from the city said.

Omicron variants BF.7 and BA.5.1.7, which are more contagious than previous strains, are the primary strains in the city, said Li Shujian, deputy director of the Shaoguan Center for Disease Control and Prevention, over the weekend.

"It was the first time the BA.5.1.7 variant had been detected on the mainland," he said.

Compared with other Omicron variants, BF.7 is characterized by stronger infectivity, ease of diffusion and ability to escape detection. It can easily cause large-scale outbreaks, Li said.

The patients mainly develop symptoms of high fever, persistent cough, body aches, headaches, sore throat, changes in sense of smell and loss of appetite after they have been infected. Experts say that BF.7 could become the main virus worldwide this autumn and winter, Li said.

Studies have indicated that BF.7 is currently the third-most common variant detected in the United States. In Europe, BF.7 is also spreading rapidly.

In Hohhot, Inner Mongolia autonomous region, more than 600 infections of BF.7 were reported in a single day. The northwestern city has reported more than 1,800 cases since Sept 28, he said.

According to Li, Shaoguan's first case was detected when a local resident surnamed Chen, who was infected with the virus outside the city, returned from Inner Mongolia on Sept 30.

Chen, who did not observe the city's anti-COVID-19 measures and policies, walked around public venues without wearing a mask and spitting. He infected several local residents and introduced a major risk of a BF.7 outbreak in the city, Li said.

Measures are now being introduced to help prevent and control the spread of the virus in the city, he added.

In Ulanhot, Inner Mongolia, a confirmed case and seven asymptomatic carriers, were detected from Thursday to Sunday, according to a statement released the city's health commission.

Gene sequencing indicated they were highly consistent with the first case of the BF.7 variant found in Hohhot on Sept 28, it said.