Omicron strain spreads with the doubling tme of 3.2—3.6 days in South Africa province of Gauteng that achieved herd immunity to Delta variant Frederic Grabowski, Marek Kochańczyk, and Tomasz Lipniacki ⋆ Department of Biosystems and Sof Mater, Insttute of Fundamental Technological Research, Polish Academy of Sciences, 02‐106 Warsaw, Poland ⋆ Corresponding author e‐mail: tlipnia@ippt.pan.pl Abstract Omicron, the novel, highly mutated SARS‐CoV‐2 Variant of Concern (belonging to the Pango lineage B.1.1.529), was frst collected on November 8, 2021, in Gauteng province of South Africa. By the end of November 2021 it has spread towards fxaton in Gauteng and was detected on all contnents. Based on data collected tll December 7, 2021, we showed the exponental growth of the Omicron variant over the four‐week period in Gauteng (November 8–December 5, 2021) with the doubling tme equal 3.38 day [CI 95%: 3.18–3.61 day]. Log–linear regression suggests that the spread began around October 10, 2021, however due to stochastcity in the inital spread this estmate is likely inaccurate. Phylogenetc analysis indicates that the Omicron strain started to diverge in between October 28 and November 5, 2021. This implies that the hidden spread of Omicron before October 10, 2021 (which would suggest slower strain growth) is unlikely. The very short doubling tme of Omicron in Gauteng, a province that has reached herd immunity to the Delta variant (implied by the decrease of the weekly number of cases between July and October, 2021, at no signifcant mobility restrictons), suggests that Omicron will cause abrupt outbreaks of COVID‐19 epidemics across the world, and will become the (temporarily) dominant strain. 1 . CC-BY 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted December 9, 2021. ; https://doi.org/10.1101/2021.12.08.21267494 doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice.