ABSTRACT
<p><b>OBJECTIVE</b>To analyse seasonal influenza epidemic situation in 2006, and to analyse the genetic and antigenic characteristics of viral hemagglutinin (HA) gene.</p><p><b>METHODS</b>The single-way hemagglutination inhibition (HI) tests were used to test the antigenic characteristics of these viruses from influenza surveillance network, and the HA1 genes were sequenced based on the antigenic test results according to different isolation times and sites.</p><p><b>RESULTS</b>The influenza virus types A and B co-circulated in 2006. influenza A H1N1 subtype and Victoria-like B influenza circulated preponderantly during this epidemic season. The HA1 gene sequence of H1N1 viruses showed that 192, 193, 196, 198 positions (located at antigenic site B) have an amino acid substitute, compared with the last circulating strain A/Hubeihongshan/53/2005(H1N1). Two amino acid changes at 142 and 144 positions compared with A/Yunnan/1145/2005 (H3N2). There was no change in influenza B viruses either Victoria-like B or Yamagata-like B virus, i.e . antigenic characteristics is analogous to B/shenzhen/155/2005 and B/tianjin/144/2005, respectively.</p><p><b>CONCLUSION</b>The H1N1 and H3N2 influenza viruses had changing antigenic and genetic characteristics in 2006. Influenza virus types B did not change in 2006.</p>