BACKGROUNDTo study the antigenic and genetic characteristics of influenza (H3N2) virus circulated in China in 2004.

METHODSSingle-way and cross-way hemagglutination inhibition (HI) tests were firstly used to determine the reactivity with the reference serum of virus isolates. Based on the serological results, virus isolates were selected according to the different time and location in China in 2004. The HA1 domain of HA gene of those virus isolates were then sequenced in order to analyze the gene characterization.

RESULTSSingle-way HI test results showed that 52.3% of isolates showed 4 folds or more HI titer difference compared to A/Fujian/411/2002 (H3N2) itself (international reference strain in 2004). Cross-way HI test results showed that the antigenic ratio was 4. The nucleic acid and amino acid sequence data of HA1 domain showed that the mutated virus appeared in early February of 2004, and became the dominant circulating strain gradually. There were four important mutant positions, they were 159 Y>F, 189 S>N, 145 K>N, 226 V>I, respectively. The results also indicated that the mutated viruses originated from southern China, then transmitted to northern China, according to the analysis of time and location distribution.

CONCLUSIONThe HA1 domain of HA gene of influenza virus (H3N2) isolated from 2004 in China showed mutation and antigenic drift, and the mutated viruses were becoming the dominant circulating strain in China, and showed amino acid sequence difference compared to A/Fujian/411/2002 (H3N2) A/Wellington/1/2004 (H3N2), the vaccine components pronounced by WHO for 2004-2005 northern hemisphere and 2005 southern hemisphere respectively, which suggested that further surveillance should be conducted to monitor the virus mutation in circulation.