Surveillance of Influenza Virus and HA1 Genetic Characterizations of Group A Influenza Viruses(H3N2) Circulated in Tianjin during 2007-2010

Objective To investigate the prevalence character of influenza and molecular characterization of influenza A(H3N2) virus HA1 gene in Tianjin in 2007-2010.Methods Influenza viruses were isolated with MDCK cells.HA1 gene segments of 14 H3N2 subtype influenza virus strains were amplified with RT-PCR,and RT-PCR products were sequenced after purification.And then the sequences identity was compared with WHO influenza vaccine strains.Finally the phylogenetic tree was constructed according to nucleotide sequences.Results A total of 4 220 throat swabs were collected,among them,894 influenza strains were isolated,positive rate was 21.2%.H1 subtype,H3 subtype and B type became predominant prevalence subtype alternately.Amino acid sequences analysis showed that continually point mutation was observed as time went on.Six point mutations occurred in the beginning of 2007,and these mutations continuous existed in the following three years.Five new point mutations were observed in 2009-2010 isolated strains.These point mutations located in antigenic determinant A and B areas.Phylogenic tree showed two branches.2009-2010 isolated strains located in a separate branch,other strains stayed in the same branch with corresponding vaccine strains.Conclusion H1 subtype,H3 subtype,novel H1 subtype and B type co-exist in Tianjin,and they are prevalent alternately.Antigenic drift occurs in H3 subtype influenza virus,and drift extent trends to increase.The prevalence level is related with the extent of antigenic drift.