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Comparative Epidemiology of Outbreaks Caused by SARS-CoV-2 Delta and Omicron Variants in China: An Observational Study
36 Pages Posted: 12 Jan 2023More...
Background: Stringent measures had been implemented in China since February 2020 to contain spread of SARS-CoV-2 until Omicron subvariants caused sustained outbreaks across the country in late 2022. We examined the epidemiology of Delta and Omicron outbreaks in China to inform policy making in control of COVID-19.
Methods: COVID-19 cases from May 2021 to October 2022 were extracted to identify outbreaks of Delta and Omicron variants in all provinces of mainland China. We examined the proportion of asymptomatic cases, stringency of government responses to outbreaks measured by population mobility (inter-provincial inflow and outflow), and indices in the Oxford Covid-19 Government Response Tracker, and estimated the time-varying daily effective reproduction number (Rt) and the duration of Rt to drop to below 1 for identified Omicron and Delta outbreaks.
Findings: Compared to the Delta variant, the Omicron variant in 2022, leading to outbreaks in 26/31 provinces with a total of 865,100 cases and 85% being asymptomatic, was associated with more frequent (4.3 vs 1.6 outbreaks per month) and longer outbreaks (mean duration: 13 vs 4 weeks per outbreak) although the average daily inter-provincial inflow and outflow were 5% (95% CI: -2%, 12%) and 5% (-3%, 12%) lower and the average stringency of containment measures was 12% (9%, 15%) higher when Omicron was predominant. The time-varying daily Rt was 0.40 (95% CI: 0.30, 0.50, p < 0.001) higher in Omicron than Delta outbreaks. On average Omicron outbreaks were suppressed in 32 days (95% CI: 26, 39), substantially longer than Delta outbreaks (14 days; 95% CI: 11, 19; p = 0.004).
Interpretation: Control measures previously effective in suppressing transmission of Delta could not contain Omicron outbreaks in China. Ongoing epidemiologic evaluation on new virus variants and interventions is critical to inform timely decision-making in response to COVID-19.
Funding Information: This project was supported in part with federal funds from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Department of Health and Human Services, under contract no. 75N93021C00016, and the Theme-based Research Scheme (Project No. T11-705/21-N) of the Research Grants Council of the Hong Kong SAR Government. BJC is supported by an RGC Senior Research Fellowship (grant number: HKU SRFS2021-7S03) and the AIR@innoHK program of the Innovation and Technology Commission of the Hong Kong SAR Government.
Declaration of Interests: Ben Cowling (BJC) reports honoraria from AstraZeneca, Fosun Pharma, GSK, Haleon, Moderna, Pfizer, Roche and Sanofi Pasteur. All other authors reported no competing interest.
Keywords: COVID-19, transmission, SARS-CoV-2, Delta, Omicron
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