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High number of Tamiflu-resistant viruses found Add to ...

Influenza experts admitted Monday they have been startled by the discovery this season of an unexpectedly high number of human flu viruses that appear to be naturally resistant to Tamiflu, the drug that countries around the world are stockpiling for use in the next flu pandemic.

The viruses have been isolated from people infected with influenza A viruses of the H1N1 subtype in a number of European countries. And Canada's National Microbiology Laboratory in Winnipeg has reported finding one such virus, in a child who is believed to have been infected with influenza in Sudan before travelling to Canada.

The World Health Organization is convening a virtual meeting of experts Tuesday to try to get a handle on how far afield the resistant virus has been found, how common it is and what the findings signify.

"I think this is a very concerning change in influenza virus resistance patterns," Dr. Frederick Hayden, a leading antiviral expert and a member of the WHO's Global Influenza Program, said from Geneva.

"This is not only interesting, it's unusual and would not have necessarily been predicted by the necessary information. So it's certainly something we're taking seriously and trying to gather additional information (on)."

The reports relate only to human flu viruses of the H1N1 subtype. Most of the cases of resistance have occurred in people who were not treated with Tamiflu (oseltamivir), a fact that suggests viruses with this mutation may be more capable of spreading from person to person than had been previously thought.

It had been believed that viruses that acquired this resistance mutation would pay for that gain with a corresponding loss in their ability to transmit - that it would make them less able to compete with unaltered viruses in the race to infect and spread among human hosts.

"It's a bit of a surprise ... because several of the pre-clinical studies would suggest that this mutation does confer a biologic cost to the virus," said Dr. Hayden. "(But) it doesn't seem to reduce fitness very much."

A spokesperson for Hoffman-La Roche, the Swiss maker of Tamiflu, cautioned against over-interpreting the findings.

"The clinical significance of these results is not yet known and more extensive surveillance is required."

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