Virtually all the 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 the chief strain 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 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.
Resistance appeared several years ago in Japan, which uses more Tamiflu than any other country, and experts feared it would spread.
But the Japanese strains were found only in patients already treated with Tamiflu, and they were "weak" — that is, they did not transmit to other people.
"This looks like a spontaneous development of resistance in the most unlikely places — possibly in Norway, which doesn't use antivirals at all," Monto said.
Dr. Henry Niman, a biochemist in Pittsburgh who runs recombinomics.com, a Web site that tracks the genetics of flu cases around the world, has been warning for months that Tamiflu resistance in H1N1 was spreading.
He argues that it started in China, where Tamiflu use is rare, was seen last year in Norway, France and Russia, then moved to South Africa (where winter is June to September), and back to the northern hemisphere in November.
This is another example of evolution in action. Mutations occur which change the DNA features of a virus, where it was resistant to drugs, is not so now.