Prevelance, genetic drift in hemagglutinin of influenza A(H3N2) viruses circulating in Shanghai Pudong district during 2015-2017

Objective To investigate the prevalence and to evaluate the genetic characteristics and the amino acid substitutions of influenza A (H3N2) virus in Shanghai Pudong district during the 2015-2017 influenza season. Methods A total of 5 436 swabs were collected from the influenza like illness (ILI) patients at two influenza sentimental hospitals in Shanghai during January 2015 to July 2017 and were used to detect influenza subtypes by real time RT-PCR. The positive samples were cultured with MDCK cells. The hemagglutinin (HA) genes of 275 strains of influenza A (H3N2) were sequenced and were compared with the HongKong circulating strain and WHO recommended vaccine strain to analyze the amino acid substitutions at key sites. Results Noticeably, approximately 22.9% (1 245/5 436) cases were tested positive, and 54.5% (678/1 245) influenza-positive patients were infected by influenza A (H3N2). The HA sequences of 275 A (H3N2) virus strains were divided into 2 clades, e.g. 3c.2a and 3c.3a and 96.0% (264/275) of strains were belong to 3c.2a clade. Sixty two out of 264 strains were belong to 3c.2a1 subclade which lies in 3c.2a clade. N121K AA substitutions were accounted to a high proportion (10.5%, 29/275). Strains carrying N121K AA substitution in 2017 were accounted for remarkably high proportion (94.1%). Conclusions Influenza A (H3N2) strains were the predominant subtype in Shanghai Pudong district during 2015-2017. The significant increment of influenza A (H3N2) were due to genetic drift caused by HA gene variants. Key words: Influenza A virus; Hemagglutinins genes; Genetic variation