Comparison of epidemiological characteristics and transmissibility of different strains of COVID-19 based on the incidence data of all local outbreaks in China as of March 1, 2022

Abstract

Background: The epidemiological characteristics and transmissibility of Coronavirus Disease 2019 (COVID-19) may undergo changes due to the mutation of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) strains. The purpose of this study is to compare the differences in the outbreaks of the different strains with regards to aspects such as epidemiological characteristics, transmissibility, and difficulties in prevention and control.

Methods: COVID-19 data from outbreaks of pre-Delta strains, the Delta variant and Omicron variant, were obtained from the Chinese Center for Disease Control and Prevention (CDC). Case data were collected from China's direct-reporting system, and the data concerning outbreaks were collected by on-site epidemiological investigators and collated by the authors of this paper. Indicators such as the effective reproduction number (R _eff), time-dependent reproduction number (R _t), rate of decrease in transmissibility (RDT), and duration from the illness onset date to the diagnosed date (D _ID )/reported date (D _IR ) were used to compare differences in transmissibility between pre-Delta strains, Delta variants and Omicron variants. Non-parametric tests (namely the Kruskal-Wallis H and Mean-Whitney U tests) were used to compare differences in epidemiological characteristics and transmissibility between outbreaks of different strains. P < 0.05 indicated that the difference was statistically significant.

Results: Mainland China has maintained a "dynamic zero-out strategy" since the first case was reported, and clusters of outbreaks have occurred intermittently. The strains causing outbreaks in mainland China have gone through three stages: the outbreak of pre-Delta strains, the outbreak of the Delta variant, and outbreaks involving the superposition of Delta and Omicron variant strains. Each outbreak of pre-Delta strains went through two stages: a rising stage and a falling stage, Each outbreak of the Delta variant and Omicron variant went through three stages: a rising stage, a platform stage and a falling stage. The maximum R _eff value of Omicron variant outbreaks was highest (median: 6.7; ranged from 5.3 to 8.0) and the differences were statistically significant. The RDT value of outbreaks involving pre-Delta strains was smallest (median: 91.4%; [IQR]: 87.30-94.27%), and the differences were statistically significant. The D _ID and D _IR for all strains was mostly in a range of 0-2 days, with more than 75%. The range of duration for outbreaks of pre-Delta strains was the largest (median: 20 days, ranging from 1 to 61 days), and the differences were statistically significant.

Conclusion: With the evolution of the virus, the transmissibility of the variants has increased. The transmissibility of the Omicron variant is higher than that of both the pre-Delta strains and the Delta variant, and is more difficult to suppress. These findings provide us with get a more clear and precise picture of the transmissibility of the different variants in the real world, in accordance with the findings of previous studies. R _eff is more suitable than R _t for assessing the transmissibility of the disease during an epidemic outbreak.

Keywords: Delta variant; Omicron variant; SARS-CoV-2; effective reproduction number; transmissibility; variant.

Figure 1

Flow chart of select outbreaks for fitting R_eff and R_t value.

Figure 2

Epidemic curve COVID-19 transmitted locally in China from March 28, 2020 to March 1, 2022.

Figure 3

Maximum R_eff and R_t value of 9 pre-Delta strains outbreaks and 13 Delta variant outbreaks and 8 Omicron variant outbreaks. (A) Maximum R_eff value. (B) Maximum R_t value.

Figure 4

Duration from the illness onset date to the diagnosed date (D_ID)/reported date (D_IR) of all pre-Delta strains, Delta variant and Omicron variant outbreaks in China from March 28, 2020 to March 1, 2022. (A,C,E) D_ID of all pre-Delta strains, Delta variant and Omicron variant outbreaks, respectively. (B,D,F) D_IR of all pre-Delta strains, Delta variant and Omicron variant outbreaks, respectively.

Figure 5

Duration of outbreak and total number of cases of all pre-Delta strains, Delta variant and Omicron variant outbreaks in China from March 28, 2020 to March 1, 2022. (A) Duration of outbreak. (B) Total number of cases.