Atlanta ? The government, for the first time, is urging doctors not to prescribe two antiviral drugs commonly used to fight influenza after discovering that the predominant strain of the virus has built up high levels of resistance to them at alarming speed.

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, as they become more commonly used.

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.