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Two Drugs Rendered Useless Against Flu Virus

There are strains circulating that resist amantadine and rimantadine, but newer antivirals still work, CDC officials say.

January 15, 2006|Thomas H. Maugh II | Times Staff Writer

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.

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

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

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

Dr. Nancy Cox, head of the 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% in some Asian countries.

Cox and Gerberding attributed the rising resistance to widespread inappropriate use of the drugs made possible because they were 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 2%, she said. Last year, it was 11%. But a study of 120 virus isolates completed Friday showed that 109 of them -- 91% -- were resistant to 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.

She said no data were available about how widely the two drugs were used.

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.

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

The most effective way to prevent spread of the flu, she said, 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 as 18 states are reporting widespread regional activity. The heaviest outbreaks to date have been in the Southwest, including California.

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

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