The spread of Tamiflu-resistant swine flu virus among four patients at Duke University Medical Centre in Durham, N. C., and five in a hospital in Wales are being investigated by epidemic experts.

Researchers say that these are the first cases of virus which is resistant to Tamiflu, spreading from person to person.

According to Duke’s Daniel Sexton, who leads the hospital’s investigation, swine flu will become even more difficult to handle if the Tamiflu-resistant virus spreads widely and causes more deaths.

The preliminary genetic evidence suggests that the virus spread among patients at the hospital but the investigations of the two hospital outbreaks are underway.

Sexton says that researchers at the Centers for Disease Control and Prevention are now testing virus samples from Duke to see whether they're identical.

Sexton says that Duke patients were located in a ward for people with cancer or severe blood disorders. All of them were severely ill and were highly susceptible to infections.

The WHO has reported 57 cases of Tamiflu-resistant virus worldwide. The Wales virus remains sensitive to the antiviral drug Relenza like the swine flu virus isolated from the Duke patients.