The initial stage ending around the 20 April is characterized by numerous transmissions from Mexico to the United States, as well as some local transmissions within Mexico and the United States (Figure 5a ). The fact that two Mexican isolates trace their ancestry back to the United States was inescapable as the oldest sequences present in the data set were all sampled in the United States. However, most ancestries of early US isolates point to a Mexican origin (Figure 5a ), which gives further support for the A/H1N1 pandemic having originated in Mexico. The second stage (ending around 5 May) is characterized by the spread of the virus within North America, with several local transmissions (dots and sunflowers on Figure 5b and Supplementary Movie S1), indicating the existence of secondary outbreaks and the likely establishment of the pathogen in several cities in the United States and Canada. During the same time period, Mexico and the United States also started seeding the rest of the world (Figure 5b ). It is worth noting that the first case of within-country transmission outside the Americas (France on 1 May, Figure 5b , Supplementary Movie S1) is observed remarkably early after the spread of the new influenza strain within the United States. In the third and last stage, we observe a considerable number of transmissions inferred from the Americas to the rest of the world, alongside with transmissions between countries outside the Americas (Figure 5c ). Local transmissions with a high statistical support point to multiple secondary outbreaks occurring in several countries, which likely reflect the worldwide establishment of the new influenza strain as a global pandemic.