Major Flu Strain Found Resistant to Leading Drug

Virtually all the dominant strain of flu in the United States this season is resistant to the leading antiviral drug Tamiflu, and scientists and health officials are trying to figure out why.

The problem is not yet a public health crisis because this has been a below-average flu season so far, and because the Tamiflu-resistant strain, one of three circulating, is still susceptible to other drugs. But infectious disease specialists are worried nonetheless.

Last winter, about 11 percent of the throat swabs from patients with the most common type of flu that were sent to the Centers for Disease Control and Prevention for genetic typing showed a Tamiflu-resistant strain. This season, 99 percent do.

“It’s quite shocking,” said Dr. Kent A. Sepkowitz, director of infection control at Memorial Sloan-Kettering Cancer Center in New York. “We’ve never lost an antimicrobial this fast. It blew me away.”

The single mutation that creates Tamiflu resistance appears to be spontaneous, and not a reaction to overuse of the drug. It may have occurred in Asia, and it was widespread in Europe last year. In response, the disease control agency issued new guidelines two weeks ago. They urged doctors to test suspected flu cases as quickly as possible to see if they are influenza A or influenza B, and if they are A, whether they are H1 or H3 viruses.

The only Tamiflu-resistant strain is an H1N1. Its resistance mutation could fade out, an agency scientist said, or a different flu strain could overtake H1N1 in importance, but right now it causes almost all flu cases in the country, except in a few mountain states, where H3N2 is prevalent.

Complicating the problem, antiviral drugs work only if taken within the first 48 hours of infection. A patient with severe flu could be given the wrong drug and die of pneumonia before test results come in. So the new guidelines suggest that doctors check with their state health departments to see which strains are most common locally and treat for them.

“We’re a fancy hospital, and we can’t even do the A versus B test in a timely fashion,” Dr. Sepkowitz said. “I have no idea what a doctor in an unfancy office without that lab backup can do.”

If a Tamiflu-resistant strain is suspected, the disease control agency suggests using a similar drug, Relenza. But Relenza is harder to take; it is a powder that must be inhaled and can cause lung spasms, and it is not recommended for children under 7.

“And we strongly suggest that people get a flu shot,” Dr. Fine said. “There’s plenty of time and plenty of vaccine.”

Exactly how the Tamiflu-resistant strain emerged is a mystery, several experts said.

The flu typically kills about 36,000 Americans a year, the C.D.C. estimates, most of them the elderly or the very young, or people with problems like asthma or heart disease; pneumonia is usually the fatal complication.

Thanks t0 Donald G. McNeil Jr. for the information for this post.
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