Antiviral resistance continues to be a clinical issue during flu season in the United States. In the 2008-2009 season, the most common influenza A strain isolated has been H1N1 (roughly 37% of specimens). About 4% of specimens have been positive for the H3N2 strain. In tests of 654 influenza A H1N1 isolates as of March 28, 2009, 99.2% were resistant to oseltamivir (Tamiflu), none was resistant to zanamivir (Relenza), and 0.5% were resistant to adamantanes (rimantadine [Flumadine] or amantadine [Symadine, Symmetrel]). Among 94 influenza A H3N2 isolates tested, none was resistant to either oseltamivir or zanamivir, but there was 100% resistance to adamantanes. None of 105 influenza B isolates tested was resistant to oseltamivir or zanamivir (CDC FluView for week ending 2009 Mar 28; accessed April 8, 2009).
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