Transmission dynamics and epidemiological characteristics of Delta variant infections in China

Abstract

Background: The Delta variant of SARS-CoV-2 has become predominant globally. We evaluated the transmission dynamics and epidemiological characteristics of the Delta variant in an outbreak in southern China. Methods: Data on confirmed cases and their close contacts were retrospectively collected from the outbreak that occurred in Guangdong, China in May-June 2021. Key epidemiological parameters, temporal trend of viral loads and secondary attack rates were estimated and compared between the Delta variant and the wild-type SARS-CoV-2 virus. We also evaluated the association of vaccination with viral load and transmission. Results: We identified 167 patients infected with the Delta variant in the Guangdong outbreak. The mean estimates of the latent period and the incubation period were 4.0 days and 5.8 days, respectively. A relatively higher viral load was observed in Delta cases than in wild-type infections. The secondary attack rate among close contacts of Delta cases was 1.4%, and 73.9% (95% confidence interval: 67.2%, 81.3%) of the transmissions occurred before onset. Index cases without vaccination (OR: 2.84, 95% confidence interval: 1.19, 8.45) or with one dose of vaccination (OR: 6.02, 95% confidence interval: 2.45, 18.16) were more likely to transmit infection to their contacts than those who had received 2 doses of vaccination. Discussion: Patients infected with the Delta variant had more rapid symptom onset. The shorter and time-varying serial interval should be accounted in estimation of reproductive numbers. The higher viral load and higher risk of pre-symptomatic transmission indicated the challenges in control of infections with the Delta variant.

Longitudinal analysis of SARS-CoV-2 vaccine breakthrough infections reveal limited infectious virus shedding and restricted tissue distribution

“…Finally, enrollment in this study concluded before the arrival of the Delta variant at either study site. It remains unclear how well the effects of vaccination on viral infection dynamics that we describe apply to Delta variant breakthrough infections, given the unique features 12 and enhanced transmissibility 13 of this variant relative to the viruses we captured here.…”

Correlation between times to SARS-CoV-2 symptom onset and secondary transmission undermines epidemic control efforts

“…53 As the mean serial interval can approximate the mean generation interval, this finding may indicate that the generation interval of the Delta variant will not vary much from that of wild-type SARS-CoV-2. Similarly, a preprint analysis of Delta variant cases from China estimated an incubation period of 5.8 days, 54 which resembles the results from a meta-analysis on wild-type SARS-CoV-2 incubation periods. 29…”

Monitoring key epidemiological parameters of SARS-CoV-2 transmission

“…Exact knowledge of these four quantities contributes to our ability to control an outbreak 1 but they can vary depending on disease-mitigating interventions 2 and population structure, as well as the inherent properties of the SARS-CoV-2 variant 3,4 . Inaccurate estimates of the four quantities can lead to incorrect estimation of the time-varying reproduction number (R t ) (ref.…”