Betweenness measures the number of shortest paths between any two vertices in a network that lie on a given node. In the context of influenza, increasing vaccinations in regions of high betweenness would hypothetically have the greatest effect on diminishing the spread of infection worldwide. This novel strategy contrasts with previous studies simulating containment only at the source of influenza [26, 27] . For H3N2, this criteria highlighted Europe and North America as promising candidates for vaccination programs ( Figure S5D ). Clustering by country revealed USA, Japan, and Australia as sites in the influenza network vulnerable to disruption ( Figure 6D ).