The antiviral drug oseltamivir (Tamiflu) is virtually useless for treating the most common strain of the influenza virus circulating this flu season, according to reports released Monday.

March 02, 2009

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By Anne Harding
MONDAY, March 2, 2009 ( — The antiviral drug oseltamivir (Tamiflu) is virtually useless for treating the most common strain of the influenza virus circulating this flu season, according to reports released Monday.

So far this flu season, 98.8% of the influenza A (H1N1) virus samples tested by the Centers for Disease Control and Prevention (CDC) are Tamiflu-resistant, up from 12.3% last year. The resistant strain is just as easily spread—and just as deadly—as other strains of the flu virus, according to new research in the Journal of the American Medical Association.

The findings shouldn’t cause panic, experts agree, because there are still drugs effective against Tamiflu-resistant influenza A (H1N1). “Doctors still have weapons against influenza,” said Anne Moscona, MD, of Weill-Cornell Medical College in New York City, who was not involved in the research. “It’s become a little more complicated, and doctors need to take a little more information into account.”

But the findings are a “warning call” underscoring the importance of prevention strategies, Dr. Moscona says, especially the need to get flu shots. “Our bottom line always is to get vaccinated for influenza and this just points to another reason why vaccination is so important,” says Nila J. Dharan, MD, of the CDC in Atlanta, the author of one of the studies. “The influenza vaccine really is our best line of defense.”

The CDC first warned about Tamiflu resistance in December 2008, recommending that nearly everyone with the flu get zanamivir (Relenza) instead. Children under 5 and people who are unable to take inhaled medications should get a combination of Tamiflu and rimantidine (Flumadine), according to the CDC.

Next page: Flu viruses are already resistant to two older drugs

The rapid development of resistance to Tamiflu was a “surprise,” but shouldn’t have been, says David M. Weinstock, MD, of Harvard Medical School in Boston, who co-authored an editorial accompanying the new studies. The exact same pattern of resistance developed against two older flu drugs, adamantine and rimantidine, he explained. In 2006, the CDC recommended doctors stop using these drugs to treat patients with the flu, and use Relenza or Tamiflu instead.

Some experts had suspected that the Tamiflu-resistant strain might be less likely to spread, and less likely to make people sick. But the new report on cases from the 2007-2008 flu season by Dr. Dharan and her colleagues shows that this is not the case. Their analysis of 1,155 influenza A (H1N1) samples from last year’s flu season found the resistant strain made people just as sick as the non-resistant strain. However, according to Dr. Dharan, “there’s nothing that makes us think that this virus is more transmissible or causes more serious disease.”

This study also showed that the resistance wasn’t due to people over-using Tamiflu, but something that just happened on its own. “There’s an innate property of the virus that confers innate resistance to this medication,” Dr. Dharan explains.

Right now, doctors have no easy way to tell if someone is infected with a flu virus that is resistant to a particular drug, although such tests could become available years down the road. Pediatricians often use rapid tests that can determine if a sick child really has the flu. Such tests aren’t typically used in adults, according to Dr. Moscona, because their symptoms are much more straightforward: they quickly get a fever and respiratory symptoms.

There is a test available to distinguish between influenza A and influenza B; if your strain belongs to the B family, Tamiflu will still work for you. But this test is rarely used, Dr. Weinstock says. (In general, three types of influenza viruses circulate every year—two A strains and a B strain.)

If you do suspect the flu, get yourself to the doctor ASAP, Dr. Moscona urges. The earlier you receive treatment, the more effective it will be.

Most importantly, get vaccinated, experts say. If you haven’t gotten a flu shot or inhaled vaccine yet, it’s not too late; the flu season won’t be over for a few more weeks. But it’s best to get immunized as soon as the shot becomes available, in October, well before the flu season is in full swing, according to Dr. Dharan.

You should also stay informed. Flu treatment recommendations have changed rapidly, and patients who keep track of them may be just as well informed as their doctors, who likely aren’t getting the information any sooner, according to Dr. Weinstock. “This is the kind of thing where patient education really could be very useful.”

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