Tamilfu is one of the antiviral drugs used to treat people infected with the H1N1 virus. (Michael Probst/Associated Press)

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, a WHO spokesperson said Thursday.

The UN health agency has not changed its assessment of the disease, WHO flu chief Dr. Keiji Fukuda told reporters.

Clusters of resistance have been discovered at hospitals in Wales and North Carolina and need to be investigated further, he said.

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.

"What it points out is there needs to be a lot of vigilance taken with those groups of patients, but it probably does not have big implications for the overall pattern of spread or the overall patterns of illness in the general community," he said from Geneva during the WHO's weekly briefing on the swine flu pandemic.

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

On Wednesday, public health officials reported one more patient has tested positive for H1N1 virus resistant to Tamiflu at a hospital in Cardiff, raising the total number of cases in that hospital to six.

It was not unexpected that more patients exposed to the original case would test positive for the resistant strain, said Dr. Roland Salmon, director of the National Public Health Service Communicable Disease Surveillance Centre in Cardiff.

"The emergence of influenza A viruses that are resistant to Tamiflu is not unexpected in patients with serious underlying conditions and suppressed immune systems, who still test positive for the virus despite treatment," Salmon said in a release.

"In this case, the resistant strain of swine flu does not appear to be any more severe than the swine flu virus that has been circulating since April."

Mutation in Norway

Last week in North Carolina, four cancer patients tested positive for a type of flu virus that was resistant to Tamiflu.

More investigation will also be needed into a mutation reported by health authorities in Norway to clarify whether the mutated virus is more likely to trigger severe illness or become more common.

"And the question is whether this mutation … suggests that there is a fundamental change going on in viruses out there or whether there is a turn for the worst in terms of the severity? And I think the answer right now is that we are not sure."

Fukuda also commented on Canada's vaccine surveillance system, which picked up a potential problem in one lot of H1N1 vaccine that was halted from further use. Before then, six people who were vaccinated from the batch developed anaphylaxis, or severe allergic reaction.

On Wednesday, the Public Health Agency of Canada confirmed 24 cases of anaphylaxis among people who have received the H1N1 vaccine.

Flu activity appears to be leveling off in some areas of the northern hemisphere, but Fukuda cautioned against saying activity has peaked until there is a definitive downward turn.

With files from The Canadian Press