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In 2006, the circulating influenza A virus rapidly became resistant to the adamantanes, prompting recommendations against their use and leaving only neuraminidase inhibitors (NAIs; e.g., oseltamivir, zanamivir) for treatment or prophylaxis. To assess the prevalence of resistance to NAIs, CDC investigators conducted a surveillance study of influenza A virus (H1N1). For the 2007–2008 influenza season, 12% of 1155 isolates from 45 states were resistant to oseltamivir. No epidemiological or clinical differences were noted between resistant and susceptible cases. No patients with resistant viruses reported prior exposure to oseltamivir. In the 2008–2009 season (through February 19, 2009), 98% of 268 isolates were resistant to oseltamivir.
In anot…