U.S. Flu Shows Resistance to Flu Drug, CDC Says

WASHINGTON (Reuters) Dec 19 - A common strain of influenza circulating in the United States this winter is resistant to Tamiflu, the most popular drug used to treat it, federal health officials said on Friday.

The situation poses little danger, according to the U.S. Centers for Disease Control and Prevention, because Tamiflu is only used in a minority of cases. It advised doctors to use rival drug Relenza or rimantadine, an older drug.

Forty nine out of 50 samples tested resist the drug, although they can still be treated with other flu medications, the CDC said in a special advisory to doctors.

"It is still very early in the season. There is very little influenza out there," CDC Director Dr. Julie Gerberding told reporters in a telephone briefing.

"This is probably actually not going to affect very many people because we don't use a lot of antiviral drugs in our country," Gerberding said. "Most people with influenza don't get any treatment."

In a normal flu season, three strains of flu circulate called H1N1, H3N2 and influenza B. Flu kills about 36,000 Americans in an average year.

It is the H1N1 strain that is turning up resistant samples, Gerberding said, and comes mostly from Hawaii, Massachusetts and Texas, the states with the most cases of influenza.

Last year, just under 11 percent of the H1N1 samples tested were resistant to Tamiflu. Gerberding said she did not think the virus had evolved, but that the strain that happened to pop up was also resistant to the drug.

"We can't predict whether or not these strains will end up being the most important strains in this year's flu season. This particular H1N1 could fizzle out," Gerberding said.

Tamiflu, known generically as oseltamivir and made by Roche AG and Gilead Sciences Inc., can both prevent and treat flu if taken quickly enough.

A similar drug is Relenza, or zanamivir, made by GlaxoSmithKline under license from Australia's Biota Inc.

The U.S. national stockpile of antivirals is about 80 percent Tamiflu and 20 percent zanamivir, according to the Health and Human Services Department. The CDC's Dr. Tim Uyeki said this season's development illustrated the need to keep a diversified array of drugs on hand.

"But zanamivir... is not approved for those less than 7 years old," Uyeki said. People with asthma are also advised not to use the drug, which is inhaled.

Gerberding noted that this year's flu vaccine matched the three strains circulating so far very well. The CDC says there is still time for Americans to get a flu shot, as the season usually peaks in February.

The CDC and the U.N. World Health Organization are concerned about the threat of a new and deadly strain of flu developing that would sweep the world. That is one reason to keep a stockpile of antivirals handy, CDC says, although this year's flu season appears to be mild.