Influenza A virus subtype H1N1
When it comes to this year's flu season, health experts are saying so far, so good. The vaccine appears to be effective against the most widely circulating strains of influenza, and there is plenty of the drug to go around. The only dark cloud has been a disturbing rise in drug-resistant flu, but new data published Monday in the Journal of the American Medical Association suggest that despite the resistant strain, things aren't as bad as they could be. (Read "What You Need to Know About Drug-Resistant Flu.")
Researchers at the Centers for Disease Control and Prevention (CDC) first reported in December that 2% of influenza strains circulating in the 2007-08 season were resistant to the most popular antiflu remedy, oseltamivir, or Tamiflu. They warned that the prevalence of these strains would probably continue to increase, and indeed, early data from the current season suggest they have. Influenza is composed of three subtypes of virus, and last year 12% of one of those subtypes, known as H1, were resistant to oseltamivir. This year almost all of the H1 contingent, 98%, are resistant. (Read "Getting Closer to a Flu Supervaccine.")
But in the new JAMA study, the CDC scientists report that infection with drug-resistant flu does not cause more severe illness or lead to more hospitalizations than infection with nonresistant strains. The study also finds that the resistant strain did not arise from overuse of Tamiflu; in fact, during the past flu season, the resistant strain was found widely in countries with low Tamiflu use, like Norway, but less commonly in places, like Japan, where Tamiflu use is high. If overuse had played a role in the emergence of the resistant strain, health officials might have recommended that clinicians restrict prescriptions of the drug. Instead, it appears that the drug-resistant strain mutated on its own. "Flu viruses mutate all the time," says Dr. Alicia Fry, a medical epidemiologist at the CDC and a co-author of the study. "We suspect this is just one of those spontaneous mutations that occurred and is not related to overuse of the drug."
The resistant strain is also treatable with antiviral medications other than Tamiflu; zanamivir, or Relenza, still combats all three subtypes of influenza. And for those for whom zanamivir is inappropriate — the drug comes as an inhaled powder and is not recommended for children under age 7 — the combination of Tamiflu and an older antiviral, Flumadine (rimantadine), can be effective in fighting the disease. (In 2006 the CDC recommended against prescribing Flumadine because another of influenza's three subtypes was resistant to the drug. But in combination with Tamiflu, the medication can be effective.) (Read "A Brief History Of: The Flu Vaccine.")
That's good news, but it also means that doctors need to be even more aware of how they treat the flu. "Treating flu is complicated, and we are worried that there will be some confusion among clinicians about what antiviral drugs to use," says Fry. Most do not have the time or ability to test each patient's virus to determine whether it is a resistant strain, so the CDC recommends that physicians stay updated on the predominant strains circulating in their communities. If the Tamiflu-resistant variety is common, they should prescribe either Relenza or the combination of Tamiflu and Flumadine instead.