The CDC announced on January 14, 2006 that the main strain of influenza virus currently circulating in the United States (H3N2, an influenza A strain) has developed resistance to amantadine and rimantadine, two common antiviral drugs used to treat and to prevent the flu. 91% of H3N2 virus samples have tested positive for resistance, and the CDC is recommending that these drugs not be used this year to treat or to prevent the flu.
Antiviral medications, when used very early in the illness, can shorten the duration of the flu and potentially lessen the severity as well. They can also be used to help prevent the development of the flu in persons exposed to the virus. They act by slowing down the ability of the virus to reproduce itself. Two classes of drugs are currently available. The adamantanes (amantadine and rimantadine) are M2 ion channel inhibitors and are effective only against influenza A strains. Amantadine has been in use since 1966 and rimantadine since 1993. The neuraminidase inhibitors oseltamivir (Tamiflu) and zanamivir (Relenza) act against both influenza A and B strains. These have been in use since 1999.
