Phylodynamics of SARS-CoV-2 transmissions in France, Europe and the world during 2020

Background Although France was one of the most affected European countries by the COVID-19 pandemic in 2020, the dynamics of SARS-CoV-2 transmissions within France, Europe and worldwide remain only partially characterized during the first year of the pandemic. Methods Here, we analyzed GISAID deposited sequences from January to December 2020 (n = 638,706 sequences). To tackle the huge number of sequences without the bias of analyzing a single sequence subset, we produced 100 independent and randomly selected sequence datasets and related phylogenetic trees for different geographic scales (worldwide, European countries and French administrative regions) and time periods (first and second half of 2020). We applied a maximum likelihood discrete trait phylogeographic method to date transmission events and to estimate the geographic spread of SARS-CoV-2 to, from and within France, Europe and worldwide. Results The results unraveled two different patterns of inter- and intra-territory transmission events between the first and second half of 2020. Throughout the year, Europe was systematically associated with most of the intercontinental transmissions, for which France has played a pivotal role. SARS-CoV-2 transmissions with France were concentrated with North America and Europe (mainly Italy, Spain, United Kingdom, Belgium and Germany) during the first wave, and were limited to neighboring countries without strong intercontinental transmission during the second one. Regarding French administrative regions, the Paris area was the main source of transmissions during the first wave. But, for the second epidemic wave, it equally contributed to virus spread with Lyon and Marseille area, the two other most densely populated cities in France. Conclusion By enabling the inclusion of tens of thousands of viral sequences, this original phylogenetic strategy enabled us to robustly depict SARS-CoV-2 transmissions through France, Europe and worldwide in 2020.