Some flu viruses in the United States and Canada have shown increased levels of resistance to the antiviral drug Tamiflu (oseltamivir), the World Health Organization (WHO) said on Friday. Similar trends have been reported earlier in Europe.
Tamiflu is currently the recommended treatment for humans infected with bird flu, and governments worldwide have been stockpiling the drug in case of a flu pandemic.
The drug resistance appears to be limited to the H1N1 variant which causes seasonal flu, while H3N2 or influenza B strains and the H5N1 strain that causes bird flu did not show increased resistance. 
—World Health OrganizationWhile previous studies showed resistance levels below 1%, samples tested by the U.S. Centers for Disease Control and Prevention now showed around 5% resistance, and in a Canadian survey of 81 samples, 8 showed resistance. The European Centre for Disease Prevention and Control found 19 cases of resistance among 148 samples from last November. Among 16 samples from Norway, 12 showed resistance. In a wider study of 437 H1N1 isolates from 19 countries, the European agency found 59 resistant samples from 9 countries.
Resistance has not been shown to increase in Japan or Hong Kong. In Japan, Tamiflu is widely prescribed for seasonal flu. The WHO is now contacting national authorities to get more data on flu resistance.
These viruses have acquired a mutation that makes them resistant, and the WHO statement said that a resistant strain has probably emerged spontaneously. The statement added that these mutations would most likely render Tamiflu ineffective for the prevention or treatment of flu caused by resistant H1N1 viruses.
The WHO also stated that this phenomenon does not influence the effectiveness of flu immunizations. Resistant strains have shown no evidence of causing more severe disease or being more easily transmissible.

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Drug-resistant flu rising, says WHO