The PCR products were purified by MicroSpin Sephacryl S-400 HR columns (Amersham Biosciences, UK). Sequencing reactions were performed using BigDyeH Terminator v3.1 Cycle Sequencing Kits (Applied Biosystems, Foster City, USA) with 2.5 mL of template cDNA. For sequencing the HA and NA genes, the primers used are shown in Table 3 .