BEIJING (REUTERS) - 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 told state media.

The Delta variant, first detected in India, is becoming globally dominant with its increased transmissibility, the World Health Organisation warned last week.

In an interview aired by China Central Television on Thursday (June 24), Dr Feng Zijian, former deputy director at the Chinese Centre for Disease Control and Prevention, did not provide further details.

Without naming the two vaccines, the researcher 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 shots in China's mass inoculation scheme are inactivated vaccines.

These include drugs from Sinovac Biotech and Sinopharm that are used in countries such as Brazil, Bahrain and Chile.

The Delta variant has caused infections in three cities in southern Guangdong, officials said, where 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 (who were) vaccinated... became severe cases," Dr Feng said.