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Tamiflu Resistant Flu Raising Concerns

Feb 5, 2008 | Parker Waichman LLP Tamiflu might not be as effective during a flu pandemic as once thought.  Flu strains in the United States, Canada, and parts of Europe are showing a higher resistance to Tamiflu, raising questions about its potential effectiveness in the event of a human bird flu pandemic.  The World Health Organization (WHO) reported elevated resistance in North America on Friday, but said it was too early to know the chances for increased Tamiflu resistance in the H5N1 strain of avian influenza.  The WHO did not change its recommendation that Tamiflu be used to treat human cases of bird flu.

A number of governments have been stockpiling Tamiflu, which is made by Switzerland's Roche Holding AG and Gilead Sciences Inc. of the U.S.  Tamiflu is recommended for use as a first line of defense in case bird flu sparks a human influenza outbreak.  Health experts fear that the virus—which now mainly affects poultry—could mutate into a form that spreads easily among people, triggering a deadly pandemic.

The WHO said it was investigating the extent of resistance worldwide to Tamiflu, known generically as oseltamivir, in some seasonal H1N1 flu viruses that have a mutation making them "highly resistant."  "The frequency of oseltamivir resistance in H1N1 viruses in the current influenza season is unexpected and the reason why a higher percentage of these viruses are resistant is currently unknown," the WHO said.

The U.S. Centers for Disease Control and Prevention (CDC) reported a five percent prevalence of Tamiflu resistance in H1N1 virus samples tested.  In Canada, eight out of 128 samples showed resistance.  "These preliminary data indicate that oseltamivir resistance in H1N1 viruses is geographically variable but not limited to Europe," said the WHO in a statement.  A preliminary survey issued by the European Centre for Disease Control (ECDC) this week said that of 148 samples of influenza A virus isolated from 10 European countries during November and December, 19 showed Tamiflu resistance; the mutated H1N1 is a sub-type of influenza A.  Of 16 samples from Norway, 12 tested positive for Tamiflu resistance.

The new "elevated resistance to oseltamivir" appears limited to seasonal H1N1 viruses and not H3N2 or influenza B, which are circulating, the WHO said.  "This means that oseltamivir would most likely be ineffective for treating or preventing infections caused by resistant H1N1 strains, although the drug will be effective against other influenza virus infections," it added.  The WHO was contacting national health authorities to determine the extent of drug resistance.  Neither Japan—where Tamiflu is widely prescribed for seasonal flu—nor Hong Kong, had seen increased resistance, it said.  Past studies found Tamiflu resistance rates ranging from zero to 0.5 percent.

Experts are looking to see if the mutated viruses found early in the winter can compete when regular flu viruses move from one vulnerable respiratory tract to the next.  "One of the great conundrums of influenza is the fact that viruses appear and disappear and nobody knows really where they go from or to," said Dr. Maria Zambon, head of the respiratory virus unit of Britain's Health Protection Agency.

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