June 29 (Bloomberg) -- Roche Holding AG said a swine flu patient treated with its Tamiflu drug in Denmark showed resistance to the antiviral medicine for the first time.

The patient was given a low dose of Tamiflu preventively after coming in contact with someone infected with the H1N1 pandemic virus, said David Reddy, who heads Roche’s influenza task force. The person developed flu symptoms and was found to have a virus mutation that evaded the drug, Reddy said on a conference call today.

The patient has recovered and doctors have found no other people carrying the resistant strain, Denmark’s National Board of Health said today in a statement on its Web site. Tamiflu studies show that 0.4 percent of adults and 4 percent of children with seasonal influenza develop resistance, according to Reddy.

“We know from seasonal flu that a proportion of patients can develop resistance,” Reddy said. “We fully expect that this also can occur during treatment with a new flu strain.”

The new virus, which has killed more than 300 people worldwide, “remains sensitive to the drug,” Reddy said. “What this does underscore is the continued need for resistance monitoring.”

Gauging Resistance

The virus hasn’t acquired new characteristics that make it resistant to Tamiflu, according to Reddy. Instead, in this one patient, when faced with a low dose of the medicine it found a way to evade it, he said. That’s known as drug-induced resistance, he said.

Roche, based in Basel, Switzerland, is studying flu sufferers in several countries to gauge drug resistance.

Swine flu hasn’t previously shown resistance to Roche’s Tamiflu or GlaxoSmithKline Plc’s Relenza, antiviral medicines that reduce the severity of the disease when administered in the first few days. The World Health Organization reported today that there have been 70,893 cases of the virus globally, including 44 people in Denmark.

“This one case of a resistant virus doesn’t change our recommendations to use Tamiflu and the treatment remains part of Denmark’s preparedness measures,” the National Board of Health said in the statement.

To contact the reporter on this story: Christian Wienberg in Copenhagen at cwienberg@bloomberg.net; Marthe Fourcade in Paris at mfourcade@bloomberg.net

Last Updated: June 29, 2009 11:52 EDT