- University of Copenhagen Denmark
Background: The Covid-19 pandemic caused by the SARS-CoV-2 virus started in China in December and has since spread globally. Many countries have instated measures to slow the spread of the virus. Information about the introduction of the virus to a country and its further spread can inform the gradual opening of a country and the avoidance of a second wave of infections. Denmark has seen first cases in late February and is currently opening up. Methods: Sequenced virus genome can be used to reconstruct transmission events. We perform a phylogenetic analysis of 742 publicly available Danish SARS-CoV-2 sequences and put them into context using sequences from other countries. Results:Our findings are consistent with several introductions of the virus to Denmark from independent sources, the majority from a ski area in Austria. We identify several chains of mutations that occurred in Denmark and in at least one case find evidence that it spread from Denmark to other countries. There is a number of the Danish mutations that are non-synonymous, and in general there is a considerable variety of strains circulating in Denmark.Conclusion:The introduction of the virus from the Austrian ski area happened after Iceland had declared this area high-risk, thereby giving an independent assessment of the effectiveness of the early identification of high-risk areas.The likely further spread of the virus from Denmark challenges the common narrative that Denmark only got infected from abroad, highlighting that Denmark was part of a network of countries among which the virus was being transmitted. Furthermore, our broad analysis of the mutations does not indicate that the virus underwent a systematic change in virulence in Denmark. We believe that the methods used can be a valuable tool to identifying and validating transmission chains during the reopening