By counting the number of indegrees and outdegrees of each node for H3N2, we identified that the tropics and the northern temperate zone ( Figure S4A ), specifically Asia and North America ( Figure Figure  S5A ), transmit and receive the most seeding events to and from the rest of the world, respectively. In a similar manner, we identified USA, Hong Kong, Australia, and China as the greatest seeders, and USA, Japan, Australia, and Hong Kong as the most seeded ( Figure 6A) .