Background: To study the incidence of adamantane resistance among influenza A (H3N2) viruses isolated from the mainland of China since 1989 through our influenza surveillance system, and to provide more information for the clinical usage of adamantane drugs.
Methods: Totally 584 influenza A (H3N2) virus strains were randomly selected from our surveillance network since 1989, the adamantane drug resistance related gene M2 of all 584 strains was sequenced, and the drug sensitivity of viruses was also assayed by using biological methods in cells.
Results: No adamantane resistant strains were detected among the strains isolated from 1989 to 1999, but there was a surprisingly increased resistance rate of 56% in 2003 compared with 3.4% in 2002, and in 2005 the resistance rate increased to 77.6%.
Conclusion: Over 50% of virus among the strains isolated showed adamantane resistance since 2003, and the incidence rate is increasing.