Introduction and transmission of SARS-CoV-2 B.1.1.7 in Denmark

Michaelsen, T. Y.; Bennedbaek, M.; Christiansen, L. E.; Jorgensen, M. S. F.; Moller, C. H.; Sorensen, E. A.; Knutsson, S.; Brandt, J.; Jensen, T. B. N.; Chiche-Lapierre, C.; Collados, E. F.; Sorensen, T.; Petersen, C.; Le-Quy, V.; Sereika, M.; Hansen, F. T.; Rasmussen, M.; Fonager, J.; Karst, S. M.; Marvig, R. L.; Stegger, M.; Sieber, R.; Skov, R. L.; Legarth, R.; Krause, T. G.; Fomsgaard, A.; Danish Covid-19 Genome Consortium,; Albertsen, M.

In early 2021, the SARS-CoV-2 lineage B.1.1.7 became dominant across large parts of the world. In Denmark, comprehensive and real-time test, contact-tracing, and sequencing efforts were applied to sustain epidemic control. Here, we use these data to investigate the transmissibility, introduction, and onward transmission of B.1.1.7 in Denmark. In a period with stable restrictions, we estimated an increased B.1.1.7 transmissibility of 58% (95% CI: [56%,60%]) relative to other lineages. Epidemiological and phylogenetic analyses revealed that 37% of B.1.1.7 cases were related to the initial introduction in November 2020. Continuous introductions contributed substantially to case numbers, highlighting the benefit of balanced travel restrictions and self-isolation procedures coupled with comprehensive surveillance efforts, to sustain epidemic control in the face of emerging variants.