The H1N1 swine flu virus can develop resistance quickly to antivirals used to treat it, U.S. doctors reported on Friday.

Government researchers reported on the cases of two people with compromised immune systems who developed drug-resistant strains of virus after less than two weeks on therapy.

Bacteria quickly develop resistance to antibiotics, which must be used carefully. Viruses can do the same and doctors worried about resistance had recommended against using antivirals for flu except in patients who really needed them.

“While the emergence of drug-resistant influenza virus is not in itself surprising, these cases demonstrate that resistant strains can emerge after only a brief period of drug therapy,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

“We have a limited number of drugs available for treating influenza and these findings provide additional urgency to efforts to develop antivirals that attack influenza virus in novel ways,” he said in a statement.

Swine flu emerged a year ago in the United States and Mexico and spread around the world in just six weeks, killing thousands of people. It hit children and young adults especially hard.

Older antiviral drugs did not work against it - they do not work against seasonal flu, either - but Roche AG’s Tamiflu, known generically as oseltamivir, did. It was not widely used, however.

Dr. Jeffery Taubenberger and colleagues studied two flu patients who had immune limitations due to past blood stem cell transplants. They were treated with Tamiflu.

Writing in the journal Clinical Infectious Diseases, Taubenberger and colleague Dr. Matthew Memoli said the virus infecting one patient developed a drug-resistant mutation after nine days and the other after 14 days of treatment.

And one of the patients also developed resistance against a second antiviral, Biocryst’s peramivir, which is an experimental drug approved for emergency intravenous use in patients who cannot take Tamiflu.

This patient continued getting worse despite 24 days on Tamiflu and was given peramivir for 10 days.

Finally, GlaxoSmithkline’s flu drug Relenza, known generically as zanamivir, did work and the patient recovered, the researchers said.

“These cases of rapid appearance of drug-resistant 2009 H1N1 influenza in immune-compromised patients are worrisome and should prompt clinicians to reconsider how they use available flu drugs,” Memoli said.

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WASHINGTON (Reuters)