Based on phylogenetic analysis of the HA and NA nucleotide sequences from 1999 to 2006 (Figure 2 ), ten isolates representative for the phylogenetic clustering of sequences from each subtype in each season, as far as possible, were included in the final HA and NA tree ( Figure 2 ) and representatives were chosen for complete genome sequencing. Generally the H3N2 HA and NA genes formed seasonal phylogenetic clusters ( Figure 2 ). However, we observed that strains of different lineages and clusters cocirculated within the same season and that viruses had reassorted with viruses from previous seasons ( Figure 2 ).