Comparative Epidemiology of Outbreaks Caused by Sars-Cov-2 Delta and Omicron Variants in China

Abstract

Background: China implemented strict measures to contain the spread of SARS-CoV-2 since 2020. However, sustained Omicron outbreaks occurred in 2022. We analyzed the epidemiology of Delta and Omicron outbreaks to inform COVID-19 control policies.

Aims of study: The objective is to analyze and compare the epidemiological features of outbreaks in China caused by Delta in 2021 and Omicron in summer 2022, with the aim of providing scientific evidence to support decision-making in response to the COVID-19 pandemic.

Methods: We extracted COVID-19 cases numbers from May 2021 to October 2022 to identify of outbreaks of Delta and Omicron variants in all provinces of mainland China, and compare their epidemiology and estimated time-varying daily effective reproduction number (Rt). To determine if our results for the province level were also the same as for the city level, we repeated the analysis using city-level data for the 43 major epidemic cities selected.

Results: Compared to Delta, Omicron caused more frequent (4.3 vs 1.6 outbreaks per month) and longer (mean duration: 13 vs 4 weeks per outbreak) outbreaks in 2022, with 865,100 cases and 85% being asymptomatic. Although the average stringency of containment measures was 12% (95% confidence interval (CI): 9%, 15%) higher, the daily Rt was 0.45 (95% CI: 0.38, 0.52, p < 0.001) higher in Omicron outbreaks. Omicron outbreaks were suppressed in 32 days (95% CI: 26, 39) on average, substantially longer than Delta outbreaks (14 days; 95% CI: 11, 19; p = 0.004). The findings obtained from the city-level analysis were comparable to those obtained from the provincial-level analysis.

Implications: Control measures previously effective in suppressing transmission of Delta could not contain Omicron outbreaks in China, highlighting the need for ongoing evaluation of epidemiology of new variants and interventions to inform COVID-19 response decisions.

Note: This conference abstract was presented at the 9th International Conference on Infectious Disease Dynamics organized by the journal Epidemics. This abstract has not been screened by SSRN for potential for public harm and should not be used to inform any clinical decision making. No competing interests or funding statements have been declared.