Antibodies triggered by two Chinese Covid-19 vaccines are less effective against the Delta variant compared with other strains, but the shots still offer protection, a Chinese disease control researcher has told state media.

The Delta variant of the coronavirus, first detected in India, is becoming the globally dominant variant of the disease with its increased transmissibility, the World Health Organisation's chief scientist warned last week.

Read: What we know so far about the Covid-19 Delta variant and its sub-lineage Delta Plus

In an interview aired by China Central Television late on Thursday, Feng Zijian, researcher and former deputy director at the Chinese Center for Disease Control and Prevention, did not provide further details.

Without naming the two vaccines, Feng said they fell into the category of inactivated vaccines, which contain "killed" coronavirus that cannot replicate in human cells.

Five out of the seven domestically developed vaccines in China's mass inoculation scheme are inactivated vaccines. These include shots from Sinovac Biotech and Sinopharm used in countries such as Brazil, Bahrain and Chile.

The Delta variant has caused infections in three cities in southern Guangdong province, officials said, where a total of 170 locally confirmed patients were reported between May 21 and June 21.

It remains unclear how many of them contracted the Delta variant.

About 85 per cent of the Guangdong cases in the latest outbreak were found in the provincial capital, Guangzhou.

"In the Guangdong outbreak [...] none of those vaccinated infections became severe cases, and none of the severe cases were vaccinated," Feng said.