The increase in amantadine- and rimantadine-resistant H3N2 influenza A viruses over the past decade is alarming, Rick A. Bright, Ph.D., of the Centers for Disease Control and Prevention, Atlanta, and his colleagues reported.
Between the 1994–1995 and 2003–2004 flu seasons, the proportion of resistant H3N2 viruses increased from 0.4% to 12.3%, a study of more than 7,000 influenza A field isolates showed. Most (84%) of the resistant viruses were isolated since the 2003 flu season, and 61% of those were from people in Asia, the investigators found (Lancet 2005;366:1175–81).
Furthermore, between the 2003–2004 season and the first 6 months of the 2004–2005 season in the United States, resistance among comparable isolates increased from 2% to 15%. The high incidence of drug-resistant H3N2 viruses circulating in regions that also have a high incidence of avian influenza (H5N1)—which is also resistant to amantadine and rimantadine—suggests that these drugs should be used with greater caution.