Vietnam has detected three cases of swine flu that were resistant to the antiviral drug Tamiflu, but an expert said there was no evidence the mutant strains had infected other people.

The three were admitted to the Hospital for Tropical Diseases in Ho Chi Minh city in late August and September and have all recovered, said Rogier van Doorn, a clinical microbiologist and doctor at the hospital.

“The viruses that were isolated when they were admitted were still sensitive (to the drug), but during treatment with oseltamivir, resistance built up,” said van Doorn, referring to the generic form of Tamiflu.

“So it was not transmission of resistant viruses, but we observed that it developed during treatment of these three patients ... we have no evidence to show that (there was further transmission of resistant viruses),” he told Reuters.

Tamiflu is made by Roche and Gilead Sciences Inc and is one of two drugs shown to work well against H1N1 swine flu.

The three drug-resistant cases were among more than 600 H1N1 patients who were treated at the hospital.

Two of the patients had very mild disease and the third, a 3-year-old child, was admitted to intensive care but made a full recovery within 10 days.

Tamiflu-resistant swine flu cases have been reported in Japan, Hong Kong, Denmark and the United States.

Flu viruses are mutation-prone and experts are not surprised that they would evolve resistance, just as bacteria develop resistance to antibiotics.

However, experts fear such resistance may spread and complicate efforts to treat victims with the approach of what is likely to be a second wave of swine flu infections as the Northern Hemisphere enters the winter season.

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HONG KONG (Reuters)