Antigenic analysis of the human Influenza A (H3N2) virus during 2012-2015 in Shanghai, China

Objective To understand the patterns of emergence and circulation of influenza A (H3N2) virus variants in Shanghai area, we investigated the genetic evolution of the hemagglutinin (HA) gene and epidemic characteristics of the influenza A(H3N2) viruses which were circulated in Shanghai, China, from September 2012 to August 2015. Methods Respiratory clinical samples were collected by clinicians from patients with influenza-like illness from three sentinel hospitals. Extracted RNA from specimens was used for influenza diagnosing and genotyping with real-time RT-PCR. Amplification, sequencing and phylogenetic analysis of HA gene were performed. Results Out of the 2 346 specimens, 273(11.26%), 58 (2.39%) and 75 (2.55%) were positive for influenza A (H3N2) viruses, 2009pdmH1N1 viruses and influenza B viruses respectively. We found that influenza activity level in Shanghai increased in summer during 2014 and 2015, and influenza A (H3N2) virus was the main subtype. Phylogenetic analysis based on the full-length HA gene of influenza A (H3N2) virus was carried out. Comparing with the reference sequences, the influenza A (H3N2) viruses predominated during Dec. 2013- Jan. 2014, Jul. 2014- Aug. 2014, and Jul. 2015- Aug. 2015 were classified as genetic clade A/Finland/385/2013-like (group 3C.3), A/Switzerland/9715293/2013-like (group 3C.3a) and A/Hong Kong/5783/2014-like (group 3C.2a) respectively. And the mutations of amino acid sequences of HA between the three epidemic strains were mainly located in antigenic epitopes and receptor binding site. Conclusion The influenza epidemic in Shanghai caused by H3N2 virus may be due to the antigenic changes of influenza A(H3N2) virus during 2012 to 2015. Key words: Influenza A (H3N2) virus; Antigenic drift