Fig. 4: Local spread of Alpha in six destination countries.

A Model vs. empirical Alpha infections. In the main plot, the empirical estimates of Alpha cases are computed by multiplying the Alpha frequency from virological investigations by the reported COVID-19 incidence at the same date—the date is indicated in the plot. Model estimates are obtained with the autochthonous model A (AM A in the plot). Gray lines show ratios of 100%, 50% and 25% between observed and predicted infections attributable to reporting. In the inset, the frequency of Alpha in France obtained from the autochthonous model B (AM B in the plot) is compared with the empirical data. In both panels, black error bars indicate the prediction interval over 500 stochastic simulations obtained with the median volume of Alpha introduction, output of the international dissemination model assuming a 7-day delay between case and infection. Dark colored bars account for the variability in the output of the autochthonous models accounting for the upper and the lower limit of the prediction interval of the Alpha introductions as given by the international dissemination model. Light colored bars account for variability in the delay from infection to case reporting (ranging from 4 days to 10 days). B Empirical Alpha infections vs average international traffic. C Comparison between the date of first introduction as predicted by the international dissemination model and the seeding time of the transmission chains survived until 31 Dec 2020, predicted with the autochthonous model A. Circles indicate medians and segment the 95% prediction interval. Colors indicate the effective reproduction number of the historical strain, \({R}_{t}\), computed from weekly mortality data (Methods). The star shows the date of first Alpha detection as a comparison. D Difference between the median delay of seeding predicted by the autochthonous model A and the same quantity in the reference case—i.e. when \({R}_{t}\) is the same in all countries and traveling fluxes do not change in time, plotted against the median \({R}_{t}\) during the period from first introduction to seeding.