The CDC announced that clinicians should not prescribe amantadine (Symmetrel) and rimantadine (Flumadine) to treat or prevent influenza during the 2005-2006 influenza season because the predominant strain of flu circulating in the United States (H3N2) shows resistance to these drugs.

The CDC tested 120 influenza A (H3N2) flu virus isolates and found that 109 (91%) were resistant to amantadine and rimantadine, a sharp increase from last year when only 11% of isolates tested were resistant, and from 2004 when only 1.9% were resistant.

The CDC noted that all H1 and H3 influenza viruses tested to date were susceptible to oseltamivir (Tamiflu) and zanamivir (Relenza). During this period, CDC recommends that oseltamivir and zanamivir be prescribed if an antiviral is needed for the treatment or prevention of influenza.