Flight traffic is widely recognized as an important determinant for the spread of seasonal and pandemic influenza at large geographic scales (Cooper et al., 2006; Ferguson et al., 2006; Germann et al., 2006; Viboud et al., 2006; Wu et al., 2009) . Thus, we evaluated the relationship between air travel passenger-flow and the number of transmissions between countries inferred by SeqTrack. Both quantities were fairly correlated (r ¼ 0.55; t-test: t ¼ 5.29; df ¼ 63; P ¼ 8.3 Â 10 À7 ), although this correlation was largely driven by the high connectivity of the United States with Mexico and Canada.