The World Health Organisation says it is investigating reports from five countries that the Mexican flu is mutating. The organisation's special adviser on pandemic influenza, Keiji Fukuda, says mutations are common, and it is not yet known if they will lead to a significant increase in the severity of the A(H1N1) virus.

The Chinese authorities say eight patients have mutated versions of Mexican flu. Mutations of the virus have also been discovered in Norway, the United States, Ukraine and Japan.

Mexican flu, swine flu, A(H1N1)
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