A small number of flu viruses resistant to Tamiflu, a top antiviral drug, have been detected in Europe, health authorities said this week.

Data from more than a dozen European countries show that Tamiflu doesn't work in about 13 percent of H1N1 viruses, the main flu strain causing illness this year. Normally, resistance levels are well below 1 percent.

"It's an unexpected finding and a signal worth watching," said Fred Hayden, a flu expert at the World Health Organization. The resistant strains most likely emerged elsewhere, but were first identified in Europe.

The strain is resistant because of a single mutation. It doesn't cause more serious disease than regular strains, and can be treated with other antivirals. But experts are worried that if the resistance becomes widespread, Tamiflu, one of the best tools for fighting flu, might become useless.

"If I had only a single drug to choose for influenza, oseltamivir (Tamiflu) is the one I would go for," said Dr. Angus Nicoll, influenza coordinator for the European Centre for Disease Prevention and Control. Tamiflu, made by Roche Holding AG, has been stockpiled by WHO and by countries around the world for possible use in a flu pandemic.

But the resistant H1N1 strains do not mean that H5N1, the bird flu many experts fear could spark a flu pandemic, will develop similar resistance.

"The chance of this happening in an H5N1 virus is not zero, but probably very rare," said Dr. Joseph Bresee, chief of epidemiology and prevention at the United States' Centers for Disease Control and Prevention. At least two Tamiflu-resistant H5N1 strains have been found in Asia in the last few years.

Experts said that relying exclusively on Tamiflu is unwise. "This is a very good reminder that we don't know what the next pandemic strain will be sensitive to," Nicoll said. "Perhaps we should have more mixed antiviral stockpiles."

At the moment, health authorities are scrambling to find out how prevalent the resistant strain is worldwide. The highest levels have been found in Norway, where nearly 70 percent of tested strains have been resistant.

Resistance varies across Europe, with Italy reporting no resistant strains, and Britain, France and Denmark all reporting low but significant percentages.

In the United States, nearly 3 percent of tested flu samples have been resistant. "We don't know right now if this is a trend on the upswing or just a small blip," Bresee said.

Laboratories worldwide are also sequencing the mutated virus to try to determine where it came from and how it developed. Usually, resistant strains arise in people who have been treated with the drug. But that's not the case here.

In Norway, none of the viruses were from people who had been treated with Tamiflu. And in Japan, where Tamiflu use is the highest in the world, no resistant viruses have been reported this year. Investigations are ongoing in other countries.

Until now, experts had also believed that if viruses developed resistance, they would be less transmissible. "That assumption appears to have been incorrect," Hayden said.

As the flu season has only just started in Europe and North America, experts will be anxiously monitoring any further spread of the resistant H1N1 strains.

None of the other circulating human flu strains have so far been found to be resistant. Public health agencies say their recommendations on Tamiflu use remain unchanged.

It's still too early to know for sure what this means," Nicoll said. "But watch this space."