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Jan. 8 (Bloomberg) -- Tamiflu, the Roche Holding AG drug for influenza, can’t fight most infections that have been diagnosed in the U.S. flu season so far, health experts said.

More than half of the flu viruses that have been analyzed in the U.S. this season are of the H1N1 strain, said Joseph Bresee, chief of influenza epidemiology and prevention at the U.S. Centers for Disease Control and Prevention in Atlanta. Virtually all the H1N1 viruses the agency has tested, 72 of 73, are Tamiflu-resistant, he said.

The emergence of Tamiflu-resistant strains that can spread easily from one person to another makes it more difficult for doctors to use drugs to treat people who are at high risk of severe flu complications, such as pneumonia, he said.

“We don’t have that many antivirals available to us and in certain populations that can be very important,” Bresee said today in a telephone interview. “There’s got to be additional antiviral medicines put in our toolkits so if resistance develops against one, we’ll have others to use.”

All the other flu strains tested by CDC, called H3N2 and B flu viruses, were susceptible to Tamiflu treatment, said Terence Hurley, a Roche spokesman. “Tamiflu is a good drug, and we’ll continue to monitor the situation,” he said today in a telephone interview.

Multiple flu strains spread around the globe each year, usually originating in Asia and moving from there to Africa, Europe and North and South America. The World Health Organization said Jan. 6 that Tamiflu, the top-selling flu drug in the world, was unlikely to stop H1N1 viruses spreading in North American and Europe this year.

Other Flu Drugs

All flu viruses can still be successfully treated with GlaxoSmithKline Plc’s Relenza, an inhaled powder. Doctors should give flu patients an older flu drug, rimantadine, which most H1N1 viruses respond to, or Relenza, the CDC said Dec. 19.

Most doctors don’t use drugs to treat flu, Bresee said, because drug treatment has to be started early in the disease and most cases get better on their own.

Bresee said the flu season is still far from its peak and that cases of other flu strains that can be treated with Tamiflu may soon outnumber resistant infections. Most of the cases of flu that the agency has analyzed have come from just two or three states, and probably don’t accurately represent the flu strains spreading around the country.

The current flu season has been starting up slowly, which gives people in the U.S. plenty of time to get a flu shot to prevent the illness, Bresee said.

“If you don’t get flu at all the issue isn’t all that important,” he said.

Flu season in the U.S. typically lasts from November through March, according to the CDC. The virus strikes 5 percent to 15 percent of the population and kills 250,000 to 500,000 people a year worldwide, according to the Geneva-based World Health Organization.

To contact the reporter on this story: John Lauerman in Boston at [email protected].

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