Objective   To identify the pathogenic strain that result in measles epidemic in Xi'an city, and to search for factors concerning measles vaccination.

  Methods   Samples of thro at swabs from measles patients were collected for isolation of measles virus; to obtain DNA from suspending liquors containing measles virus by the phenol-chloroform extraction method; to amplify 450 nucleotide segments(bp)in carbon-ter minal of N-gene by reverse transcription polymerase-linked reaction(RT-PCT), and to identify the sequence and genotype of the amplified segments.The neutralization antibody levels for measles vaccine virus and wild measles virus in sera of measles patients in acute phase and convalescent phase were determined by trace neutralization test.

  Results   A total of 13 samples of throat swabs were collected, with 8 strains of positive measles virus isolated, the isolation rate was 61.5%;all of isolated virus were H1a gene subtype of measles virus.

  Conclusion   H1 genotype H1a subtype was the strain that resulted in epidemic of measles in Xi'an City.At the same time, reduction in immunity against measles and existence of immunization space areas caused by ineffective vaccination during the work of measles could not be excluded.