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GENEVA (Reuters) - The main seasonal flu virus in the United States and Canada as well as parts of Europe shows higher resistance to the antiviral drug Tamiflu, raising questions about its potential effectiveness in a human bird flu pandemic.
The World Health Organization (WHO) reported the elevated resistance in North America on Friday, but said it was too early to know what the chances may be for increased Tamiflu resistance in the H5N1 strain of avian influenza.
It did not change its recommendation that Tamiflu be used to treat human cases of bird flu.
A number of governments have been stockpiling Tamiflu, made by Switzerland's Roche Holding AG and Gilead Sciences Inc of the United States, for use as a first line of defense in case bird flu sparks a human influenza outbreak.
Health experts fear that the virus, which now mainly affects poultry, could mutate into a form that spreads easily among people and trigger a deadly pandemic.
The WHO said it was investigating the extent of resistance worldwide to Tamiflu, known generically as oseltamivir, in some seasonal H1N1 flu viruses that have a mutation making them "highly resistant."
"The frequency of oseltamivir resistance in H1N1 viruses in the current influenza season is unexpected and the reason why a higher percentage of these viruses are resistant is currently unknown," the WHO said.
The U.S. Centers for Disease Control and Prevention has reported a five percent prevalence of resistance to Tamiflu in samples of H1N1 virus tested. In Canada, 8 out of 128 samples showed resistance, roughly 6 percent, WHO spokeswoman Gregory Hartl said.
"These preliminary data indicate that oseltamivir resistance in H1N1 viruses is geographically variable but not limited to Europe," the WHO said in a statement.
A preliminary survey issued by the European Centre for Disease Control (ECDC) this week said that of 148 samples of influenza A virus isolated from 10 European countries during November and December, 19 showed signs of resistance to Tamiflu.
The mutated H1N1 is a sub-type of influenza A.
Of 16 samples from Norway, 12 tested positive for resistance against Tamiflu, according to the ECDC study.
The new "elevated resistance to oseltamivir" appears limited to seasonal H1N1 viruses, and does not involve H3N2 or influenza B viruses which are also circulating, the WHO said.
"This means that oseltamivir would most likely be ineffective for treating or preventing infections caused by these resistant H1N1 strains, although the drug will be effective against other influenza virus infections," it added.
The WHO said it was contacting national health authorities to determine the extent of resistance to the drug. Neither Japan -- where Tamiflu is widely prescribed for seasonal flu -- nor Hong Kong had seen increased resistance to date, it said.
"It is still early in the (seasonal flu) season, we don't have a full picture yet," Hartl said.
Past studies had found Tamiflu resistance rates ranging from zero to 0.5 percent, according to the U.N. agency.
Reporting by Stephanie Nebehay; Editing by Caroline Drees