The Centres for Disease Control and Prevention (CDC) has advised clinicians not to prescribe two common antivirals (amantadine and rimantadine) to treat or prevent influenza during the 2005-2006 influenza season. Laboratory testing by CDC on the predominant strain of influenza (H3N2) currently circulating in the United States shows that it is resistant to these drugs.

The alert about antiviral pills amantadine and rimantadine applies to the seasonal influenza, not the H5N1 avian flu strain which experts fear could mutate and cause a global pandemic.

CDC has tested 120 influenza A (H3N2) virus isolates and found that 109 (or 91 per cent) were resistant to amantadine and rimantadine. This represents a sharp increase from last year when only 11 per cent of isolates tested were resistant and 1.9 per cent were resistant the year before that, states a CDC release.

"This is certainly unexpected news as we now have to remove a few tools from our tool box that we use to combat influenza. Thankfully we still have antivirals available that work but this new development serves as a reminder of the importance of getting people vaccinated to prevent them from getting influenza in the first place," said CDC Director Dr. Julie Gerberding.

During this period CDC recommends oseltamivir (Tamiflu) and zanamivir (Relenza) be prescribed if an antiviral medication is needed for the treatment or prevention of influenza.

Every year in the United States, 5% to 20% of the population gets the flu; more than 200,000 people are hospitalized from flu complications, and; and about 36,000 people die from flu, according to the CDC.

Older people, young children, and people with certain health conditions, are at high risk for serious flu complications.