Relative instantaneous reproduction number of Omicron SARS-CoV-2 variant with respect to the Delta variant in Denmark

Abstract

The Omicron variant of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has become widespread across the world in a flashing manner. As of December 7, 2021, a total of 758 Omicron cases were confirmed in Denmark. Using the nucleotide sequences of the Delta and Omicron variants registered from Denmark in the GISAID database, we found that the effective (instantaneous) reproduction number of Omicron is 3.19 (95% confidence interval [CI]: 2.82-3.61) times greater than that of Delta under the same epidemiological conditions. The proportion of Omicron infections among all SARS-CoV-2 infections in Denmark was expected to exceed 95% on December 28, 2021, with a 95% CI from December 25 to December 31, 2021. Given that the Delta variant or variants less transmissible than Delta are dominant in most countries, the rapid increase in Omicron in the virus population may be observed as soon as the Omicron is introduced. Preparing proactive control measures is vital, assuming the substantial advantage of the transmission by Omicron.

Keywords: SARS coronavirus; epidemiology; evolution.

Figure 1

Estimated frequencies of Omicron and Delta variants in Demark from November 1, 2021 to January 8, 2022. Circles and triangles represent the observed frequency of sequences assigned to Omicron and Delta in the GISAID database from November 1 to December 9, 2021. Solid lines indicate the maximum likelihood estimates of frequencies of Omicron (red) and Delta (blue) variants. Dashed lines represent predicted frequencies of Omicron (red) and Delta (blue) in Demark from December 10, 2021 to January 8, 2022. Dotted lines show 95% confidence interval lower and upper bounds of estimated and predicted variant frequencies

Figure 2

Population average of $R_{\mathrm{RI}}$ of variants circulating in Demark with respect to Delta from November 1, 2021 to January 8, 2022. The solid line (until December 9) and dashed line (from December 10) indicate the maximum likelihood estimates, and dotted lines represent its 95% confidence interval