We analysed data for influenza field isolates that were obtained worldwide and submitted to the WHO Collaborating Center for Influenza at the US Centers for Disease Control and Prevention between Oct 1, 1994, and Mar 31, 2005. We used pyrosequencing, confirmatory sequence analysis, and phenotypic testing to detect drug resistance among circulating influenza A H3N2 (n=6524), H1N1 (n=589), and H1N2 (n=83) viruses.
More than 7000 influenza A field isolates were screened for specific aminoacid substitutions in the M2 gene known to confer drug resistance. During the decade of surveillance a significant increase in drug resistance was noted, from 0·4 % in 1994–1995 to 12·3 % in 2003–2004. This increase in the proportion of resistant viruses was weighted heavily by those obtained from Asia with 61 % of resistant viruses isolated since 2003 being from people in Asia.
Our data raise concerns about the appropriate use of adamantanes and draw attention to the importance of tracking the emergence and spread of drug-resistant influenza A viruses.