Japanese and Vietnamese researchers report evidence of bird flu resistance to a drug that is a main defense against it, Tamiflu. They found a drug resistant version of the bird flu in a Vietnamese teenager.

The scientists analyzed the genes of the virulent H5N1 bird flu virus taken from a 14-year-old Vietnamese girl, who became infected in February, but recovered. Their laboratory examination showed that the virus had a genetic mutation that makes it resistant to Tamiflu.

The study raises concern, because Tamiflu is the main drug nations are stockpiling to defend themselves against the virus, which is spreading westward from its epicenter in Southeast Asia. So far, it infects mainly birds.

About 140 people have become sick since it first appeared in Hong Kong in 1997, and about half of them have died. They are believed to have caught it from the fowl. But public health officials fear that the virus could mutate to a form that jumps easily among people, setting off a potentially deadly pandemic that could kill millions.

Despite the new study, a spokesman for the U.S. government's Centers for Disease Control, David Daigel, says the class of drugs to which Tamiflu belongs, called neurominidase inhibitors, is still useful.

"Right now, it is important to note that there is really no evidence that the H5N1 virus is generally becoming resistant to neurominidase inhibitors," he said. "We've seen this one influenza virus isolated from a human who was treated with Tamiflu, but this is one case, so far."

The new study underscores this point. It found that a neurominidase drug, called Relenza, worked in ferrets that they had infected with the Tamiflu-resistant virus. The researchers say this points out the need for governments to stockpile Relenza, as well as Tamiflu.

Although Mr. Daigel of the Centers for Disease Control says there is no reason to panic over the study, he says public health authorities must keep a careful watch for drug-resistant bird flu.

"It's going to be very important for us in the future to keep testing H5N1 viruses from both humans and animals, and to watch for resistance to anti-viral drugs," he said. "This is something that is going to have to be assessed and checked very carefully as we move forward."

Tamiflu and its drug cousins are the main line of defense against bird flu, because the virus has become highly immune to the other major group of flu drugs, amantadine. Experts believe that is because Asian farmers gave it to their chickens to prevent infection.