Epidemiological and transmission characteristics comparison analysis between Delta and Omicron variants of the SARS-CoV-2 (BA.2.1, BA.2.2, BA.2.76, BA.5.2) (preprint)

Since the outbreak of coronavirus disease 2019 (COVID-19), the virus has undergone three mutations, with Delta and Omicron being the most affected.This study aimed to understand the epidemiology and transmission differences between the Delta and Omicron variants, and to analyze the infection characteristics of different variants, providing a scientific theoretical basis for prevention and control strategies.We conducted a comparative analysis by selecting six local outbreaks of the Delta variant that occurred in Hunan Province in July 2021 and six local outbreaks of different sub-lineages of the Omicron variant that occurred in 2022. The results showed that asymptomatic cases were more prevalent in Omicron variant infections, with BA.5.2 having the highest proportion. The Delta and Omicron variants have identical median incubation periods of 2–3 days. In terms of secondary situations, the secondary attack rate of the Delta variant is 0.85%, while that of the Omicron variant is 1.69%. For specific Omicron subvariants, Omicron BA.2.1 has a secondary attack rate of 0.89%, Omicron BA.2.2 is 0.71%, Omicron BA.2.76 is 2.51%, and Omicron BA.5.2 has a secondary attack rate of 4.63%. The predominant mode of exposure for cases with recurrent infections of the Delta variant is cohabitation, while for Omicron variant outbreaks, cohabitation remains predominant, followed by spatial proximity and dining together.The Delta variant and the Omicron variant are both make it prone to causing multiple generations of cases in a short period, leading to a wider impact. The secondary attack rates of Omicron and Delta variants in this study were much lower than in other countries, indicating that strengthening personnel control and social regulations are beneficial for the prevention and control of newly emerging severe infectious diseases. Meanwhile, the exposure types of Omicron variant secondary cases were more diverse, and the symptoms of infected individuals were milder, indicating its greater stealthiness. Therefore, it is crucial to focus on virus mutations, strengthen surveillance, and increase prevention and control efforts if enhanced transmissibility of the variant is detected.

Transmission heterogeneities, kinetics, and controllability of SARS-CoV-2 (preprint)

A long-standing question in infectious disease dynamics is the role of transmission heterogeneities, particularly those driven by demography, behavior and interventions. Here we characterize transmission risk between 1,178 SARS-CoV-2 infected individuals and their 15,648 close contacts based on detailed contact tracing data from Hunan, China. We find that 80% of secondary transmissions can be traced back to 14% of SARS-CoV-2 infections, indicating substantial transmission heterogeneities. Regression analysis suggests a marked gradient of transmission risk scales positively with the duration of exposure and the closeness of social interactions, after adjusted for demographic and clinical factors. Population-level physical distancing measures confine transmission to families and households; while case isolation and contact quarantine reduce transmission in all settings. Adjusted for interventions, the reconstructed infectiousness profile of a typical SARS-CoV-2 infection peaks just before symptom presentation, with ~50% of transmission occurring in the pre-symptomatic phase. Modelling results indicate that achieving SARS-CoV-2 control would require the synergistic efforts of case isolation, contact quarantine, and population-level physical distancing measures, owing to the particular transmission kinetics of this virus.