Skip to main content

Viral infection and transmission in a large, well-traced outbreak caused by the SARS-CoV-2 Delta variant

Our take —

This case series, available as a preprint and thus not yet peer-reviewed, attributed 167 SARS-CoV-2 Delta variant cases to one index case between May 21 and June 18, 2021 in Guangdong, China. Although selection criteria for SARS-CoV-2 cases from early 2020 for comparison are not specified, the authors found that among transmission events outside of the same family, there was a median of 4 days from Delta variant exposure to PCR test positivity vs. 6 days in 2020 (pre-Delta) suggesting the potential for rapid and pre-symptomatic transmission with the Delta variant.

Study design

Case Series

Study population and setting

This case series follows SARS-CoV-2 infections in Guangdong, China directly attributed to the first local Delta variant SARS-CoV-2 infection in the region on May 21, 2021. From May 21 to June 18, 2021, researchers performed contact tracing to determine exposure information for positive cases and their close contacts, which included frequent PCR testing following exposure. Transmission pairs included a recipient that was a close contact and had a clear and direct epidemiologic link with the donor and did not have any contacts with other cases. Viral genomes were sequenced to further ascribe linkage between cases and outbreak parameters in a subset of cases, such as the time between exposure and first detectable viral load and viral load measurements on the day of SARS-CoV-2 detection, which were compared to the SARS-CoV-2 outbreak in the same region in early 2020.

Summary of Main Findings

The study found 167 cases epidemiologically or genetically attributed to the index case from May 21 to June 18, 2021. After removing cases that occurred within family-transmission pairs, they found that the time from the exposure to the first positive PCR test for Delta variant cases (n = 34, median 4 days, interquartile range [IQR] 3-5 days) was shorter than during the 2020 pandemic (n = 29, median 6 days, IQR 5-8 days). Among a subset of individuals (n = 62 for the Delta variant, n = 63 for early 2020), they also found higher relative viral loads as measured by PCR cycle threshold (Ct) on the first day of a positive SARS-CoV-2 test (24 with an IQR of 19-29 for Delta cases versus 34 with and IQR of 31-36 for early 2020 cases).

Study Strengths

Through extensive contact tracing and viral genetic analysis, this study presents essential information on how quickly the SARS-CoV-2 Delta variant can spread in a community.


The authors do not describe how they selected early 2020 SARS-CoV-2 comparison cases, which makes them difficult to interpret. They also report extensive population PCR testing, but do not elaborate on how frequently close contacts were tested for SARS-CoV-2 or how similar those protocols were during the comparison period. If, for example, they did more frequent testing during their study period, they may find a shorter interval between exposure and PCR-positivity due to more frequent testing. Furthermore, they do not clarify whether or not PCR Ct values were run on the same or different machines, or whether they normalized Ct values to positive controls to account for variability that occurs across PCR runs, further limiting comparisons both within and across SARS-CoV-2 transmission periods.

Value added

This study provides evidence of how rapidly the SARS-CoV-2 Delta variant can spread through a community.

This review was posted on: 1 September 2021