OBJECTIVETo study the prevalence of influenza viruses from April 2002 to June 2003.

METHODSSubtypes were identified by HI test, HA gene was amplified by RT-PCR and the nucleotide sequences of HA gene were analyzed.

RESULTSDuring the period of April 2002 to June 2003, a total number of 16 135 throat swab specimens were collected in 23 provinces, metropolitan areas and municipalities with 1 113 strains identified as influenza viruses positive, by the Chinese National Influenza Center. The rate of isolation was 6.9%. Of the 1 113 strains, 66 strains were influenza A-H1N1, 544 strains were influenza A-H3N2 viruses while 98 and 405 strains belonged to B/Yamagata and B/Victoria viruses with the rates of isolation on each subtype were 5.9%, 48.9%, 8.8% and 36.4%, respectively. Majority of influenza A (H3N2) viruses were isolated in the epidemic period of December 2002 and January 2003 as well as in the non-epidemic period. Different to influenza A-H3N2, most of the B/Victoria viruses were isolated in winter. The HA1 gene of B type strains belonged to B/Victoria lineage. Sequence analysis indicated that there were variations in influenza type in this epidemic season.

CONCLUSIONSAlthough influenza A subtype H1N1 and H3N2 as well as B type viruses were co-circulating in our country, the influenza A (H3N2) and B/Victoria isolates were found the predominant strains in the period of this study which were different from the previous years. The fact that the increase of proportion on B/Victoria and its widespread in many areas suggested there had been a change of epidemiological features caused by influenza viruses in China. It seemed important to pay more attention to the variation of HA gene in influenza A type (H3N2) strains.