Fig. 3: Timing of first importation and silent spread as estimated by the international dissemination model.

A Cumulative number of countries with an Alpha introduction as predicted by the model. The quantity is computed from the median predicted date of introduction in each country with 95% prediction interval obtained over n = 500 simulations. B, C Median date of first introduction when occurring before Dec 31 (vertical dashed line) for each country estimated by the model with 95% prediction interval computed over n = 500 simulations. For each country, we report the date of first Alpha detection (i.e. collection of first submitted sequence) (light pink) and the date of the first ever collected Alpha sequence (dark pink) from the data. For El Salvador, Papua New Guinea and Madagascar, no Alpha sequence had been reported before June 2021. D Duration of silent spread in days vs sequencing coverage. The distribution of the durations of silent spread is reported in Supplementary Fig. 3. Duration of silent spread is computed as the difference between the median date of first detection and the median date of first introduction as predicted by the model. We restricted the analysis to countries for which both first introduction and first detection were predicted to occur before 7 Jan 2021. Dashed line represents least-squares linear regression. P-value is computed from Wald test with t-distribution.