Four patients at Duke University Medical Centre patients in Durham, N. C., and five in a Welsh hospital are under investigation by epidemic experts as the anti-viral Tamiflu resistant swine flu virus clusters spread from person to person.

In the event, the Tamiflu-resistant virus spreads widely, Daniel Sexton, who is leading the hospital's investigation says, the flu will become harder to treat, and could well cost more lives.

The two hospital outbreaks are underway, with preliminary genetic evidence suggesting the virus spread among patients at the hospitals, as all four patient involved have the same resistance pattern. The Centres for Disease Control and Prevention (CDC) are now testing virus samples from Duke for similarities and to see whether they are identical, which seems to be the case as suggested by the resistance pattern.

According to Sexton, all Duke patients, severely ill and highly susceptible to infections, were located in a ward for people with cancer or severe blood disorders.

Falling ill with flu in October, on their failure to respond to Tamiflu a couple of weeks ago, specimens of their virus were sent to find out whether they might be resistant to anti-viral treatment. The results prompted Duke to launch an investigation, with three CDC epidemiologists assisting in the investigation, alongside infectious disease experts from the hospital and the state health department. In addition, the research team is also trying to determine whether the Tamiflu-resistant flu strain is circulating elsewhere in North Carolina.

Three Duke patients have already dies, whilst, the fourth who is extremely ill seems to be responding to Relenza, another antiviral treatment.

In Wales, five Tamiflu-resistant swine flu cases have been confirmed in one ward of an unidentified hospital. Three more ward patients are being tested for the drug-resistant virus; while a ninth patient is infected with the virus that appears to be still susceptible to Tamiflu.

So far, the Tamiflu-resistant swine flu virus isolated from Duke hospital patients and the Wales virus remain sensitive to anti-viral drug Relenza.