Federal and state health researchers reported Monday that the predominant strain causing the flu in the U.S. this season is resistant to Tamiflu, an anti-viral drug used to treat the illness.

Researchers from the Centers for Disease Control and Prevention and four state health departments said more than 98% of influenza A H1N1 strains that have been tested so far this season are resistant to Tamiflu, which is made by Roche Holding AG (RHHBY). The findings are set for publication in this week's Journal of the American Medical Association.

Influenza can be caused by more than one strain of the flu, including influenza B strains as well as another group of A strains that fall under the H3N2 category. There's no evidence those strains have developed resistance to Tamiflu.

In December, the CDC first alerted doctors of increasing resistance to Tamiflu among H1N1 strains of the flu, which first started causing most U.S. flu illness. Doctors said patients could receive another anti-viral drug, GlaxoSmithKline PLC's (GSK) Relenza, or be given a combination of Tamiflu and a generic drug called rimantadine.

Researchers said as of Feb. 19, resistance to Tamiflu had been identified among 264 of 268, or 98.5%, of U.S. influenza A H1N1 viruses tested. During the 2007-2008 influenza season about 12% of H1N1 strains circulating in the U.S. were resistant to Tamiflu.

"At the present time, it is unclear why [Tamiflu]-resistant influenza A H1N1 viruses emerged during the 2007-2008 season and appear to continue circulating during the 2008-2009 season," researchers wrote. They said there's a need to develop new anti-viral drugs as well as tests that can more quickly detect the types of flu causing particular illnesses.

Roche didn't immediately respond to a request for comment.

The CDC said Tamiflu could be given alone if local circulating flu viruses were likely to be H3N2 or influenza B viruses. In December and January more than 90% of flu cases in the U.S. were attributed to an H1N1 influenza strain that's likely to resist Tamiflu treatment.

But in recent weeks a larger proportion of flu illnesses have been caused by B strains, with nearly a third of illnesses in the U.S. attributed to a B strain in the week that ended Feb. 21.

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com

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