Mutating flu strains have rapidly rendered two widely used antiviral drugs ineffective.

Newer Drugs Available

The restriction is unlikely to have significant practical effects, however, because the two drugs already were being supplanted by two newer drugs, oseltamivir (Tamiflu) and zanamivir (Relenza), which are more effective and have fewer side effects, particularly in the elderly.

Fortunately, the seasonal flu viruses have not yet developed resistance to either of these drugs, said Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention.

Excessive Use of Flu Drugs May Trigger Rapid Mutation

But in an unusual Saturday news conference, she cautioned against unnecessary and inappropriate use of the newer drugs, which would increase the likelihood of resistance developing.

Dr. Nancy Cox, head of CDC's infectious diseases branch, said resistance to the older drugs had been increasing in developing countries, with studies last winter showing resistance rates as high as 70 percent in some Asian countries. She and Gerberding attributed the rising resistance to widespread inappropriate use of the drugs made possible because they are available without prescription in many countries.

But CDC experts were not expecting the rise in resistance in the U.S. "to be quite so dramatic," Gerberding said.

Two years ago, the resistance rate in the United States was only 2 percent, she said. Last year, it was 11 percent. But a study of 120 virus isolates completed Friday showed that 109 of them -- 91 percent -- were resistant to both amantadine and rimantadine.

"The lesson here is that flu constantly evolves, and we are always just one mutation away from a resistant virus," Gerberding said.

Tamiflu Available to High-Risk Groups

There is no data available about how widely the two drugs are used, Gerberding noted. Their primary applications are in the treatment of patients hospitalized with flu symptoms and as a preventive measure in group situations, such as nursing homes, where the virus can easily spread.

She said there was more than enough Tamiflu available for use in such situations, but that CDC would make the drug available from its stockpiles if spot shortages develop.

Vaccination Still Best Weapon

But the most effective way to prevent spread of the flu, she added, is vaccination, and sufficient supplies of the vaccine are available for everyone who desires it.

Flu season has started slowly this winter, according to the CDC, but activity is now picking up. The heaviest outbreaks to date have been in the Southwest.

In a typical winter, influenza kills about 36,000 people in the United States and an estimated 200,000 are hospitalized because of its effects.