But the virus remains sensitive to other medications

Health authorities in Denmark, Japan and Hong Kong have reported the appearance of H1N1 (swine flu) viruses which are resistant to the antiviral drug Tamiflu.

The strain was found in three seperate patients who did not have severe disease and have since all recovered. The viruses, while resistant to oseltamivir (or Tamiflu), remain sensitive to zanamivir.

'These instances of drug resistance appear to represent sporadic cases of resistance,' said the World Health Organisation (WHO). '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,' said the health body.

Close to 1000 pandemic H1N1 viruses have been found by scientists around the world. All the other viruses are sensitive to treatment via the antiviral drugs.

The WHO said it would continue to conduct ongoing monitoring of influenza viruses for antiviral drug resistance.