A medical worker collects swab samples from a resident for COVID-19 nucleic acid test in Liwan District of Guangzhou, south China's Guangdong Province, May 27, 2021. Photo:Xinhua

The average incubation period of the Delta variant strain (B.1.617.2) reported in South China's Guangdong Province in May was 4.4 days, shorter than the 5.2 days reported in Wuhan, Central China's Hubei Province, according to a weekly report of China Centers for Disease Control and Prevention (CDC).

The China CDC weekly report, which was published on Tuesday, showed the key transmission parameters of the highly infectious variant found in the latest local COVID-19 epidemic cluster in Guangdong. 

There were a total of 167 locally transmitted cases related to the outbreak in four cities in Guangdong as of June 23. The cases have been found to share the same gene sequence as the Delta variant strain which was first detected in India, the report said. 

From the time Guangdong's first COVID-19 local case was confirmed on May 21, the infection had led to 5 generations of transmission in the 10 days that followed. 

The average transmission time between generations was 2.9 days, noted the report. 

The average incubation period was 4.4 days, shorter than the 5.2 days reported in Wuhan. The 5.2-day was disclosed by Chinese researchers in a study published in The New England Journal of Medicine in March, 2020. The CDC weekly report also said that 64.7 percent of transmissions in Guangdong occurred during the pre-symptomatic phase. 

The percentage is also higher than the one reported in the previous outbreak in Central China's Hunan Province, where researchers found pre-symptomatic phase transmission events occurring in 59.2 percent of the more than 1,000 potential infectors. 

Guangdong announced to lift travel restrictions and open indoor activities on Saturday as the virus has been brought under control with no cases being reported the last ten days as of Friday.