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The H1N1 virus has shown resistance to Tamiflu in an Alberta woman, the second time in Canada that the antiviral drug has not worked.

Antiviral drugs are effective against swine flu if taken within days of the first symptoms. But some people have become sick despite taking them.

The case in Alberta was a lab-confirmed H1N1 infection, and the woman later recovered. She had mild symptoms. But a blood sample taken in mid-August while the woman was being treated for another condition showed the virus was still in her system and that it was resistant to Tamiflu. Further testing at the National Microbiology Laboratory in Winnipeg confirmed that the virus was Tamiflu-resistant.

"This is significant because oseltamivir or Tamiflu is the medication that is most commonly used to treat influenza around the world," said Gerry Predy, senior medical officer of health. "Any resistance to medication is important because it could possibly lead to further cases of resistant disease and potentially impact how a disease spreads."

But he cautioned that Canadians shouldn't be alarmed.

The woman, who isn't being identified for confidentiality reasons, never had to be admitted to hospital.

Dr. Predy said that despite this case, health officials are not changing the way they deal with H1N1. "It really doesn't change our approach to vaccination," he told reporters.

An investigation is under way to determine if the organism was drug resistant at the start of the patient's illness or became so during treatment.

There have been a handful of Tamiflu-resistant cases around the world. Over the summer, a 60-year-old Quebec man became Canada's first. He was given Tamiflu after his son fell ill with the virus, but still got sick. The man recovered without complications.

Canadian health authorities have stressed that Tamiflu has a strong safety profile. David Butler-Jones, Canada's chief public health officer, has said it's not a cure for the flu, but a "strong and important" treatment option.

Health officials are bracing for a second wave of the H1N1 virus this fall. Dr. Butler-Jones has indicated that a vaccine slated to arrive in mid-November could be ready as early as next month if the swine flu virus quickly turns more severe.