A second shortcoming stems from biases in the number of sequences from different regions and different seasons [8] . Most isolates of H3N2 and H1N1 were sampled from North America, whereas Africa and South America have been largely neglected [23] . Many sequences were obtained within the last 15 years, making reliable tracking over long periods of time problematic. On the level of climate zones, the number of temperate isolates far outstrips the tropics. Although hemagglutinin (HA), the HA1 domain, and neuraminidase (NA) have the most globally representative distributions of sequences, even these remain skewed ( Figure S1 , Figure S2 ).