Professor Tang Xiaoping and Li Feng’s team from the 8 affiliated hospitals of Guangzhou Medical University collaborated with Academician Zhong Nanshan and Professor Chen Ruchong’s team from the State Key Lab of Respiratory Disease and the Guangzhou Institute of Respiratory Health, the First Affiliated Hospital of Guangzhou Medical University, to accurately depict the transmission, viral kinetics and clinical characteristics of Delta through the combination of epidemiology and viral genome sequencing for the first time, in response to the “COCID-19 Epidemic on May 21” caused by Delta. Combined with clinical resources, they depicted the clinical characteristics and viral kinetics of cases in the epidemic from various angles. Recently, the research results were published on E Clinical Medicine, a sub-journal of the international renowned journal Lancet.

According to previous epidemiological investigation and viral nucleic acid genome sequencing, the “COCID-19 Epidemic on May 21” was caused by Delta and has a clear chain of transmission. The research tracked and fully reported the clear chain of transmission of this epidemic for the first time. The epidemic began with a 75-year-old woman who was exposed to the infection accidentally. She later infected another 3 people through close family gatherings or meals. Researchers found that the main transmission in the epidemic was through direct and indirect contact. Among them, 30.8% of the infected were spread through meals, followed by family gatherings (30.13%), community transmission (18.59%), and other means of transmission including work and social contact (19.87%).

Meanwhile, researchers included those infected in the “COCID-19 Epidemic on May 21” into the group of patients infected with the wild strain of COCID-19 in the 2020 pandemic for research, and analyzed the clinical characteristics and viral kinetics of the “COCID-19 Epidemic on May 21” from various angles, to summarize the characteristics of Delta and seek the risk factors for progression to severe disease.

Researchers found that the incubation period of Delta is short and the transmission speed is fast. The median incubation period is only 4.7 days, which is significantly shorter than that of the wild strain (6.3 days). Further subgroup analysis showed that in the non-critical group, the incubation period (4.0 days) of Delta was significantly shorter than that of the wild strain (6.0 days). Delta can be transmitted for 4 generations within 10 days, and the fastest intergenerational transmission is less than 24 hours. Compared with the wild strain, the viral load of those infected with Delta was significantly higher (the median CT value of the former was 34.0, and that of the latter was 20.6. The lower the CT value, the higher the viral load), and the time to achieve negative nucleic acid testing of those infected with Delta is also significantly longer than that of wild strains.

Researchers also found that infection with Delta is a risk factor predicting that the disease will turn into a critical state. Among elderly patients with COVID-19 aged 60 and over, the Delta group has a 1.45 times higher risk of becoming critical than the group of wild strain; and infection with Delta develops into a critical state 2.98 times faster than the wild strain.

Researchers believed that this research revealed the characteristics of Delta, which has a shorter incubation period, faster transmission, higher viral load, longer time to achieve negative nucleic acid testing, and more likely to develop into a critical state. Therefore, it is very important to quickly track, quarantine and find people infected with Delta in time, implement timely management and control of key places, and implement nucleic acid testing of people in areas where Delta is found under special circumstances. Quickly discovering the source of infection and cutting off the transmission has provided the “Guangzhou Experience” for the prevention and control of Delta nationwide.

Original paper：https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(21)00409-0/fulltext