WASHINGTON 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.
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.
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 CDC suggested doctors use Relenza or a combination of Tamiflu and an older drug called rimantadine if infection with H1N1 is suspected.
Roche spokesman Terry Hurley noted that the CDC findings were based on a small number of samples from a limited number of states.
"It's early in the flu season and flu is unpredictable. Each flu season is unique. There is no guarantee that this situation will continue throughout the flu season. A lot can change. Last year, for example, the season started with the H1N1 virus and switched to H3N2," Hurley said by e-mail.
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.
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.
(Editing by Alan Elsner and Howard Goller)