Spatiotemporally clustering the complete H3N2 and H1N1 coding sequences for HA and NA allowed the determination of multiple statistically significant seeding seasons between 1988 and 2009. For our initial analysis, we clustered sequences into three climate zones-northern temperate, tropical, and southern temperate. To determine seasonal boundaries, we defined the northern temperate season to last from 1 st July to the 30 th June of the following year and the southern temperate season to last from 1 st January to the 31 st December of the same year [11] . Although the tropics do not have a well-defined seasonal pattern, we determined a consensus tropical flu season from 1 st October to 30 th September of the next year (Text S1, Table S1 ).