People walk past the temporarily closed 300-meter SEG Plaza in Shenzhen, South China's Guangdong Province on May 24, 2021. Photo: CFP

Seven asymptomatic cases have been reported in Shenzhen, South China's Guangdong Province in recent days. The mutated strain of B.1.1.7 variant virus, which was first detected in the UK, was identified in five of the infections. It indicates that they were related to cases imported from abroad and may share the same source or the same chain of transmission, the Shenzhen authorities announced on Tuesday.

The chain of virus transmission in Shenzhen is believed to be associated with port operations. Seven environmental samples from key areas came positive with coronavirus, said the authorities. 

The outbreak is still within a controllable range and the risk of mass spread is low, said Tuesday's announcement. As close contacts are still under medical observation, sporadic cases cannot be ruled out.

Since the recent outbreak, 450 people have been identified as having direct or secondary contacts and 159,917 people were tracked as key contacts. Apart from the announced positive infections, no other cases have been found yet.