Transmission dynamics and epidemiological characteristics of SARS-CoV-2 Delta variant infections in Guangdong, China, May to June 2021

Abstract

Background The Delta variant of SARS-CoV-2 had become predominant globally by November 2021. Aim We evaluated transmission dynamics and epidemiological characteristics of the Delta variant in an outbreak in southern China. Methods Data on confirmed COVID-19 cases and their close contacts were retrospectively collected from the outbreak that occurred in Guangdong, China in May and June 2021. Key epidemiological parameters, temporal trend of viral loads and secondary attack rates were estimated. 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. Mean estimates of latent and incubation period were 3.9 days and 5.8 days, respectively. Relatively higher viral load was observed in infections with Delta than in infections with wild-type SARS-CoV-2. Secondary attack rate among close contacts of cases with Delta was 1.4%, and 73.1% (95% credible interval (CrI): 32.9–91.4) of the transmissions occurred before onset. Index cases without vaccination (adjusted odds ratio (aOR): 2.84; 95% CI: 1.19–8.45) or with an incomplete vaccination series (aOR: 6.02; 95% CI: 2.45–18.16) were more likely to transmit infection to their contacts than those who had received the complete primary vaccination series. Discussion Patients infected with the Delta variant had more rapid symptom onset compared with the wild type. The time-varying serial interval should be accounted for in estimation of reproduction numbers. The higher viral load and higher risk of pre-symptomatic transmission indicated the challenges in control of infections with the Delta variant.

Infectious viral shedding of SARS-CoV-2 Delta following vaccination: A longitudinal cohort study

“…Thus, we were unable to control for this as a potential confounder. Previous studies have indicated that infections with Delta lineage viruses have higher peak viral loads [30,45] and longer duration of shedding than pre-Delta lineages [46,47]. This suggests that we may have underestimated differences between vaccination groups, bringing our results from early infection in line with those from Puhach et al (2022) [30].…”

Estimating the incubation period of SARS-CoV-2 Omicron BA.1 variant in comparison with that during the Delta variant dominance in South Korea

“…The mean and percentiles of incubation period of Omicron BA.1 variants were found considerably shorter than those of cases during Delta dominance period, as well as previous estimates based on other historical SARS-CoV-2 strains [ 8 , 20 ]. Given the pre-symptomatic transmission feature of SARS-CoV-2 infection [ 4 ], a shorter incubation period indicated the Omicron BA.1 cases are likely to have a relatively higher rate at which they become new sources of infection to other susceptible individuals.…”

Clinical Time Delay Distributions of COVID-19 in 2020–2022 in the Republic of Korea: Inferences from a Nationwide Database Analysis

“…This result is consistent with the finding of a previous study, which revealed that pre-symptomatic transmission contributed to 37% of all SARS-CoV-2 transmission in the country [ 27 ]. Furthermore, the estimated mean serial interval in Period 2 (4.43 days), when the Delta variant was dominant, was shorter than that in Period 1 (5.24 days), implying more rapid and intense transmission of the Delta variant [ 28 ]; this agrees with the results of other studies, revealing a shorter serial interval for the Delta variant than for the wild-type virus in the Republic of Korea [ 4 , 21 , 22 ].…”