Abstract: Objective To study the prevalence and variation of influenza A (H1N1) viruses isolated in Qingdao from 2012 to 2014.Methods A total of 2 100 throat swabs were collected from patients with influenza-like illness in two sentinel hospitals in Qingdao from April 2012 to March 2014.Influenza A (H1N1) subtype was determined with reverse transcriptase-polymerase chain reaction (RT-PCR) and then inoculated into Madin-Darby canine kidney (MDCK) cells.The mutations of gene and amino acid locus were analyzed through the whole genome sequencing of hemagglutinin (HA),neuraminidase (NA),and matrix protein (MP) segments from some influenza A (H1N1) isolates.Results Of 136 strains isolated,51 influenza A (H1N1)virus strains were selected for sequencing.The phylogenetic tree of HA gene and NA gene revealed 3 clades of the viruses isolated.Analysis of amino acid residues for HA protein showed that there were mutations involving 15 amino acid sites in antigenic determinant region,receptor binding site (RBS) and other sites in some strains.Although the residues at the enzyme active sites of NA protein were strictly conservative,compared with the vaccine strains,10 residues were substituted in the transmembrane region,membrane anchor region,and drug/antibody binding region.One strain was observed to be resistant against oseltamivir and two strains against oseltamivir and zanamivir.The sequence analysis of M2 protein showed that all of the isolated A H1N1 viruses were resistant to amantadine drugs.Conclusion All the A H1N1 influenza virus prevalent in Qingdao from 2012 to 2014 presented continual variations inducing antigenic drift.All the isolations were adamantane-resistance,but most of the strains were susceptible to inhibitors of neruaminidase.