BACKGROUND: A novel variant of SARS-CoV-2, the Delta variant of concern (VOC, 
also known as lineage B.1.617.2), is fast becoming the dominant strain globally. 
We reported the epidemiological, viral, and clinical characteristics of 
hospitalized patients infected with the Delta VOC during the local outbreak in 
Guangzhou, China.
METHODS: We extracted the epidemiological and clinical information pertaining to 
the 159 cases infected with the Delta VOC across seven transmission generations 
between May 21 and June 18, 2021. The whole chain of the Delta VOC transmission 
was described. Kinetics of viral load and clinical characteristics were compared 
with a cohort of wild-type infection in 2020 admitted to the Guangzhou Eighth 
People's Hospital.
FINDINGS: There were four transmission generations within the first ten days. 
The Delta VOC yielded a significantly shorter incubation period (4.0 vs. 6.0 
days), higher viral load (20.6 vs. 34.0, cycle threshold of the ORF1a/b gene), 
and a longer duration of viral shedding in pharyngeal swab samples (14.0 vs. 8.0 
days) compared with the wild-type strain. In cases with critical illness, the 
proportion of patients over the age of 60 was higher in the Delta VOC group than 
in the wild-type strain (100.0% vs. 69.2%, p = 0.03). The Delta VOC had a higher 
risk than wild-type infection in deterioration to critical status (hazards ratio 
2.98 [95%CI 1.29-6.86]; p = 0.01).
INTERPRETATION: Infection with the Delta VOC is characterized by markedly 
increased transmissibility, viral loads and risk of disease progression compared 
with the wild-type strain, calling for more intensive prevention and control 
measures to contain future outbreaks.
FUNDING: National Grand Program, National Natural Science Foundation of China, 
Guangdong Provincial Department of Science and Technology, Guangzhou Laboratory.