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.
Doctors give these drugs soon after patients test positive for the flu – they aren't flu vaccines, reports CBS News correspondent Joie Chen.
"Whenever there is an outbreak, especially among nursing homes – those are our most vulnerable populations – and that's where we us some of these antiviral drugs," Dr. Gigi El-Bayoumi of George Washington University told Chen.
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.
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 to fight influenza.
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."
That's especially worry with fears that bird flu could become turn into a human epidemic. The bird flu spreading through Asia infects people relatively rarely, but officials worry that it might morph into a form that spreads more easily, triggering a worldwide super-flu outbreak.
The CDC said it planned to alert doctors throughout the country via its emergency Health Alert Network and through a special edition of its weekly journal, the Morbidity and Mortality Weekly Report.
Each year, the flu kills about 36,000 people, and some 200,000 are hospitalized because of it in the United States, the CDC said. As of Dec. 31, the latest CDC data available, flu activity was only considered widespread in seven U.S. states, mainly in the Southwest and West: Texas, New Mexico, Arizona, Colorado, Utah, Nevada and California.