Both patients, who had weakened immune systems from chemotherapy and stem-cell transplants, developed flu strains resistant to the anti-viral drug oseltamivir, sold as Tamiflu.

There was no connection between the cases and no threat to the public, said Dr. Jeff Duchin, chief of communicable-disease control for Public HealthSeattle & King County. None of the patients’ family members, or the health workers who cared for them, contracted the disease.

The Seattle cases bring to 11 the worldwide total of people diagnosed with Tamiflu-resistant infections of the H1N1 virus.

"These are rare, isolated types of phenomena," Duchin said. "It’s not something we expect is going to lead to widespread drug resistance in the community."

But the cases underscore the need for judicious use of anti-viral drugs to ensure resistant strains do not become more common, he said.

Last month, CDC officials warned summer camps to stop the practice of handing out Tamiflu to healthy children in an effort to ward off infection.

"Anti-viral drugs should be used to treat an infection after it’s occurred — once the cat is out of the bag," Duchin said. "The more promiscuous use ... the more chances we have to develop resistant viruses."

Patients with weakened immune systems are most vulnerable because their bodies are not able to join forces with the drugs to fight off the virus. That means the virus persists longer in the presence of the anti-viral drugs, which is a recipe for the evolution of drug-resistant bugs.

One of the patients is a teenage boy who received a stem-cell transplant in May. He contracted the flu virus later that month and has since recovered.

The second patient, a woman in her 40s who relapsed after an earlier stem-cell transplant, remains hospitalized.

Genetic analysis showed that over several weeks of Tamiflu treatment, the flu bugs in their bodies mutated and were no longer sensitive to the drug.

The account of the cases, published in CDC’s Morbidity and Mortality Weekly Report, identifies the hospitals as Seattle Children’s and the University of Washington Medical Center.

Local and state health officials are working with the CDC to boost monitoring for drug-resistant flu viruses.

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