Molecular Epidemiological Characteristics Of H3N2Subtype Influenza Virus Strains From Fujian Province,1996-2011

Influenza virus strains of H3N2subtype have been sporadically isolated in seasonal influenza specimens from Fujian during the past surveillance seasons. In this study, the surveillance of influenza was described in Fujian, sixty-four H3N2strains from1996to2011were selected to track evidence of virus evolution, address characterization of molecular variation, and monitor protection effect of influenza vaccines on general population in Fujian. Virus stocks were amplified by inoculation in MDCK cells or chicken embryos, identified and subtyped by HA and HI assays. Fragments of HA1genes were amplified by RT-PCR, and sequenced. Molecular epidemiological characterization and circulation trends were obtained by analyzing the sequence data with bioinformatical and statistical software.According to the surveillance of influenza, it showed two peaks distributed in winter-spring and summer in Fujian, and the abnormal increase of ILI%may suggest a high risk of the pandemic of influenza. Dominant circulating influenza virus strains in Fujian were alternatively switching among H1N1, H3N2subtypes and type B strains, in1996-2011. Our results on genetic evolution indicated that H3N2subtype influenza viruses in Fujian from1996were divided into two distinct clades, i.e., the1996-2000clade and the2002-2011clade. In each clade, sequence variation rates were proportional to time, therefore, a significant antigen shift occurred in H3N2strains from Fujian around2000-2002, based on preliminary analyses. A total of91sites in nucleotide variation was identified in the HA1region, resulting25sites variation of amino acid residuals scattered in five antigen determinants. Seven variations occurred near the receptor binding sites (RBS),59variations occurred neither within antigen determinants nor the RBSs, including13unique sites associated with seasonal transition of influenza viruses. Compared with vaccine strains recommended by WHO, effective protection was demonstrated in1996-1999and2005-2009, while poor and delayed protection was observed in2000-2004. Analyses of HA1protein structure also indicated that potential glycosylation sites were increasing in1996-2011, while codons for the five disulfide-bond-forming sites were conservative and contributed for the stability of HA proteins.This study comprehensively analyzed the molecular epidemiological characterization of H3N2influenza strains from Fujian in1996-2011, particularly elucidated their HA1genes and virus evolution, provided scientific evidence for prevalence prediction, tracking transmission sources, vaccine selection, efficient control and prevention of influenza, as well as fundamentals in further molecular biological studies of influenza viruses in Fujian.