Objective To analyze the prevalence status and the genetic characterizations of influenza B viruses isolated in Hunan Province after pandemic influenza A (H1N1) 2009,and to explore possible reasons for the prevalence.MethodsThroat swabs were collected from outpatients with influenza-like illness in 23 sentinel hospitals of Hunan Province in 2010.Influenza viruses were isolated with Madin-Darby canine kidney (MDCK) cells and identified by haemagglutination inhibition test.The genomes of 10 selected influenza B viruses were sequenced and analyzed for phylogenetic and molecular characterization.ResultsWith the reduction of isolation of pandemic influenza A (H1N1)2009 viruses,influenza B virus became the predominant isolated strain in the first half of 2010.Epidemic viruses mainly belonged to the B/Victoria lineage,and both two lineages co-circulated.Seven out of 11 influenza outbreaks caused by type B.Ten strains were filled into 2 branches of BV and BY which were classified by their lineage types in polymerase (PB2,PB1,PA),hemagglutinin (HA),neuraminidase (NA),NB,membrane protein (M1),influenza B virus membrane protein M2 (BM2),and non-structural protein (NS1,NS2) phylogenetic trees except the NP phylogenetic tree in which 10 strains were all in the BY branch.Compared with World Health Organization (WHO) vaccine strains,the amino acid identity of 11 proteins of the 10 strains was high (97.2％-100.0％).However,some amino acid point mutations were found.No mutation was found in drug resistance mutation sites.Some mutations in NA,NB,PB1,PB2 and NS2 molecules were found in 2 strains isolated from outbreaks compared with strains from sentinel surveillance.Conclusions The point mutations,insertions and genetic reassortment indicate viruses sustaining evolution,which is probably the reason for predominant influenza B viruses after pandemic influenza A (H1N1) 2009 in Hunan Province.