WHO has been informed by health authorities in Denmark, Japan, and the Special Administrative Region of Hong Kong, China, of the appearance of H1N1 viruses which are resistant to the antiviral drug oseltamivir (known as Tamiflu) based on laboratory testing.

These viruses were found in three patients who did not have severe disease and all have recovered. Investigations have not found the resistant virus in the close contacts of these three people. The viruses, while resistant to oseltamivir, remain sensitive to zanamivir.

Close to 1000 pandemic H1N1 viruses have been evaluated by the laboratories in the Global Influenza Surveillance Network for antiviral drug resistance. All other viruses have been shown sensitive to both oseltamivir and zanamivir. WHO and its partners will continue to conduct ongoing monitoring of influenza viruses for antiviral drug resistance.

Therefore, based on current information, these instances of drug resistance appear to represent sporadic cases of resistance. At this time, there is no evidence to indicate the development of widespread antiviral resistance among pandemic H1N1 viruses. Based on this risk assessment, there are no changes in WHO's clinical treatment guidance. Antiviral drugs remain a key component of the public health response when used as recommended.