The models of nucleotide substitution that best fitted each dataset were determined with MEGA v5 [9] , and were: GTR+I model for HA genes, and T92+G model for NA genes, respectively. Phylogenetic relationships were reconstructed by the Neighbor-Joining method [10] , with the appropriate models of nucleotide substitution for each dataset (as described above) and bootstrap analysis of 1000 replicates, as incorporated in MEGA v5 [9] . Outgroup sequences for HA and NA genes corresponded to strain A/Puerto Rico/8/1934.