CDC Warns Against Using Certain Antivirals for Flu

After tests revealed that about nine out of 10 influenza A viruses circulating this flu season are resistant to the antivirals amantadine and rimantadine, marketed as Symmetrel and Flumadine, respectively, the CDC has issued an alert advising health professionals to avoid using these drugs for prophylaxis or treatment of flu for the remainder of the 2005-06 season.

The Jan. 14 health alert recommends using the neuraminidase inhibitors oseltamivir (Tamiflu) and zanamivir (Relenza) as alternatives to the two adamantanes, which make up the M2 ion channel inhibitor class.

"This is certainly unexpected news, as we now have to remove a few tools from our tool box that we use to combat influenza,” said CDC Director Julie Gerberding, M.D., in a press release on the announcement. “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."

Viral resistance to adamantanes can emerge rapidly during treatment because a single point mutation at one of several amino acid positions of the M2 protein can confer cross-resistance to both drugs without compromising the virus' transmissibility, according to the health alert.

Through Jan. 12, the CDC had tested 120 influenza A (H3N2) viruses isolated from patients in 23 states. Of those isolates, 109 (91 percent) were found to contain an amino acid change conferring resistance to both amantadine and rimantadine. Three influenza A (H1N1) viruses also have been tested and demonstrated susceptibility to these drugs.

All influenza viruses circulating in the United States that the CDC has screened for antiviral resistance demonstrated susceptibility to the neuraminidase inhibitors. The CDC will continue to test influenza isolates for antiviral resistance throughout the 2005-06 flu season, updating its recommendations as needed.