Comparative epidemiology of outbreaks caused by SARS-CoV-2 Delta and Omicron variants in China

Abstract

From 2020 to December 2022, China implemented strict measures to contain the spread of severe acute respiratory syndrome coronavirus 2. However, despite these efforts, sustained outbreaks of the Omicron variants occurred in 2022. We extracted COVID-19 case numbers from May 2021 to October 2022 to identify outbreaks of the Delta and Omicron variants in all provinces of mainland China. We found that omicron outbreaks were more frequent (4.3 vs. 1.6 outbreaks per month) and longer-lasting (mean duration: 13 vs. 4 weeks per outbreak) than Delta outbreaks, resulting in a total of 865,100 cases, of which 85% were asymptomatic. Despite the average Government Response Index being 12% higher (95% confidence interval (CI): 9%, 15%) in Omicron outbreaks, the average daily effective reproduction number (R_t) was 0.45 higher (95% CI: 0.38, 0.52, p < 0.001) than in Delta outbreaks. Omicron outbreaks were suppressed in 32 days on average (95% CI: 26, 39), which was substantially longer than Delta outbreaks (14 days; 95% CI: 11, 19; p = 0.004). We concluded that control measures effective against Delta could not contain Omicron outbreaks in China. This highlights the need for continuous evaluation of new variants' epidemiology to inform COVID-19 response decisions.

Keywords: COVID-19; Delta; Omicron; SARS-CoV-2; transmission.

Figure 1.

Illustration of COVID-19 epidemics in mainland China. (a,b) Symptomatic and asymptomatic COVID-19 local cases during May–December 2021 (Delta variants dominated) and during February–October 2022 (Omicron variants dominated). The solid blue line indicates the proportion of asymptomatic cases to the number of total cases on the day. (c) Proportion of province in outbreak according to symptomatic cases and total cases in mainland China during May 2021–October 2022. (d) Total vaccination dose, people received at least one dose, and ≥ 2 doses (full vaccination) per hundred persons in mainland China from May 2021 to October 2022.

Figure 2.

Heat map of case numbers by province during May 2021–October 2022. Blue brackets indicate the period of Delta outbreaks. Red brackets indicate the period of Omicron outbreaks.

Figure 3.

Heat map of changes in inter-provincial inflow and Government Response Index by province during May 2021–October 2022. Colour bars represent changes in the normalized index. Blue brackets indicate the period of Delta outbreaks. Red brackets indicate the period of Omicron outbreaks. Brown dashed lines indicate the period of National Day and Chinese New Year.

Figure 4.

Estimation of time-varying effective reproduction number using daily reported symptomatic and total cases. (a) Epidemic description and estimated reproduction numbers by province. (b,c) Time-varying reproduction number; the x-axis indicates the time of outbreak progressed. The dashed line in panel (c) indicates the time difference in the end of an outbreak estimated using symptomatic cases and the total cases.

Figure 5.

Comparison of major provincial outbreaks in 2021 and 2022. Total number of cases, peak daily case number, range of time-varying effective reproduction number (R_t), and days for R_t to drop below 1 are shown. Provinces were selected based on having outbreaks in both 2021 and 2022, with Inner Mongolia having two outbreaks in 2021. The R_t was estimated based on all cases.