The isolated cases of Tamiflu-resistance in swine flu patients in Britain and the United States likely aren't a sign that the virus is becoming resistant to the antiviral drug, WHO flu chief Dr. Keiji Fukuda said Thursday, because the cases occurred in severely immunocompromised patients, who are at risk of developing drug resistance.

What health officials are on the lookout for are signs that resistant strains of virus are moving from patients to infect those with healthy immune systems, and there is no reason to suggest that has happened, Fukuda said.

So far, 75 cases of Tamiflu resistance related to the H1N1 influenza A virus that causes swine flu have been reported to WHO.

The World Health Organization says isolated cases of drug-resistant swine flu in Britain and the United States have not changed the agency's assessment of the disease.

It says Tamiflu remains highly effective against the vast majority of H1N1 cases.