More than 98 percent of one of the influenza A viruses circulating this winter is now resistant to the antiviral drug Tamiflu, up from less than 1 percent just two years ago, according to a study in TheJournal of the American Medical Association.
"It's really hard to use Tamiflu now in any significant way; there is so much resistance out there," Andrew Pekosz, an associate professor at the Johns Hopkins University Bloomberg School of Public Health, said yesterday.
Alternative antivirals come with their own resistance issues and side effects. "Tamiflu is probably the best and perhaps the only really useful antiviral we can use against influenza right now," Pekosz said.
If prescribed within 48 hours of the onset of symptoms, antiviral medicines can reduce the severity of the flu and shorten its duration by about a day.
But public health officials say the increasing resistance to antiviral drugs makes flu prevention even more important: "Vaccination ... is the best prevention for illness due to influenza infection," said Dr. Nila J. Dharan, epidemic intelligence officer at the Centers for Disease Control and Prevention and an author of the JAMA study.
Public health authorities say the flu season has not peaked. Vaccine is still available and can provide protection within two weeks.
"Since late January or early February, we have definitely been seeing more cases ... not only of people who are getting a formal diagnosis of flu, but who are coming in with ... influenza-like illnesses," said Dr. John M. Cmar, an infectious disease specialist at Sinai Hospital of Baltimore.
Maryland health officials have reported two deaths because of the flu. Ninety-one adults and 95 children have been hospitalized.
The CDC reports that two-thirds of the flu viruses typed this season have been A-viruses, and 90 percent of those have been the newly resistant H1N1 strain.
The CDC study compared 92 patients infected last year with the flu virus resistant to oseltamivir, or Tamiflu, with 182 patients whose virus responded to the drug.
"None of the patients in our study had any known exposure to oseltamivir, or to someone close to them who was exposed to oseltamivir," Dharan said. There did not seem to be a link between Tamiflu use and the resistance that emerged.
The drug-resistant virus seemed to behave just like its drug-sensitive twin, attacking the same sorts of people, with no significant difference in their illnesses.
Physicians got the bad news about Tamiflu in December, along with new CDC guidelines on which antiviral drugs still work against which flu viruses. They were already coping with resistance among B-type viruses and another A-type flu bug, H3N2, to another class of antiviral medicines called adamantanes. Tamiflu still works for those.
"The message in our paper is for physicians to do the best they can to stay abreast of their local and state surveillance data, to know which viruses are circulating," Dharan said.
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