Shenzhen, south China's Guangdong Province, completed gene sequencing of all COVID-19 cases reported since June 14, and the results showed that four local cases had been infected with the Delta variant, local health authorities said on Tuesday.

The variant is homologous with the positive cases imported from international flight CA868, authorities added. 

A total of 18 close contacts and 101 sub-contacts of the confirmed cases have all been placed under quarantine for medical observation, and the nucleic acid test results are all negative, local health authorities said.