A very simple model to account for the rapid rise of the alpha variant of SARS-CoV-2 in several countries and the world

Since its first detection in the UK in September 2020, a highly contagious version of the coronavirus, the alpha or British variant a.k.a. B.1.1.7 SARS-CoV-2 virus lineage, rapidly spread across several countries and became the dominant strain in the outbreak. Here it is shown that a very simple evolutionary model can fit the observed change in frequency of B.1.1.7 for several countries, regions of countries and the whole world with a single parameter, its relative fitness f, which is almost universal f ≈ 1.5. This is consistent with a 50% higher transmissibility than the local wild type and with the fact that the period in which this variant takes over has been in all the studied cases around 22 weeks.