Print

Email story

Comment

Email editor

Health and medicine

CDC: Two often-used flu drugs won't help against new strains

By wire services
Published January 15, 2006

ATLANTA - Federal officials said Saturday that doctors should not prescribe two commonly used antiviral drugs, amantadine and rimantadine, for the prevention or treatment of influenza because the drugs are not effective against flu strains now circulating.

A whopping 91 percent of virus samples tested by the Centers for Disease Control and Prevention this flu season proved resistant to rimantadine and amantadine, a huge increase since last year, when only 11 percent were.

The discovery adds to worries about how to fight bird flu should it start spreading among people. Health officials had hoped to conserve use of two newer antiviral drugs, Tamiflu and Relenza, because they show activity against bird flu, unlike the older drugs.

Now, because of the resistance issue, the newer drugs are being recommended for ordinary flu, increasing the chances that resistance will develop more rapidly to them, too.

The newer drugs work against Type A and B influenza strains; the older ones work only against Type A, but cost less and are available in generic form.

CDC officials took the unusual step of calling a Saturday news conference to announce that the predominant strain this season - the type A H3N2 influenza strain - was resistant to the older drugs.

"Clinicians should not use rimantadine and amantadine . . . because the drugs will not be effective," said CDC director Dr. Julie Gerberding.

She said the lab tests, which CDC scientists had been analyzing since Friday, surprised health officials and the health agency rushed to get the word out.

"I don't think we were expecting it to be so dramatic so quickly this year," Gerberding said. "We just didn't feel it was responsible to wait three more days during a holiday weekend to let clinicians know."

The CDC tested 120 influenza A virus samples from the H3N2 strain and found that 109 were resistant to the two drugs. Two years ago, less than 2 percent of the samples were resistant. Last year, 11 percent were.

Gerberding said the agency didn't know how the resistance occurred, saying it may have been the result of a mutation in the virus or overuse of the drugs abroad, such as in countries that permit the drugs to be purchased without a prescription.

One flu expert, Dr. William Schaffner of Vanderbilt University, said the development was disconcerting, as flu now has joined the ranks of other diseases, such as tuberculosis and HIV, that recently have acquired the ability to resist front-line medications.

But Schaffner said doctors have other options.

The CDC said that all H3 and H1 influenza viruses the agency has tested so far are susceptible to the newer antivirals: Tamiflu, also known as oseltamivir, and Relenza, also called zanamivir. Doctors also recommend an annual flu shot to help prevent getting influenza in the first place.

"Tamiflu is now readily available everywhere - in most places, it is the primary antiviral being used" against flu, Schaffner said. "But we're always a bit frustrated when one of the therapeutic agents is foreclosed. It makes every infectious disease doctor worry a little bit."

[Last modified January 15, 2006, 01:49:03]