Two girls given antiviral drugs in an effort to protect children at a summer camp from the new pandemic swine flu developed resistant virus, U.S. health officials reported on Thursday.

The U.S. Centers for Disease Control and Prevention (the CDC) said this development supports its new recommendations that flu drugs not be given to prevent infection among people who are otherwise healthy.

Both girls recovered without becoming seriously ill, but the incident shows that it is easy for the new pandemic H1N1 virus to develop resistance to flu drugs, officials said.

“We are concerned about the appropriate use of antivirals,” the CDC’s Dr. Anne Schuchat said in an interview.

Two antiviral drugs work well against H1N1 swine flu - Roche AG and Gilead Science Inc’s Tamiflu, known generically as oseltamivir, and GlaxoSmithKline and Biota’s Relenza, known generically as zanamivir.

There are two older flu drugs, amantadine and rimantadine, but seasonal influenza developed resistance against them and they are no longer recommended for use alone against influenza.

Last year, the seasonal H1N1 flu virus - a different strain from H1N1 swine flu - developed resistance against Tamiflu in the United States and many other countries. Flu viruses are mutation-prone and experts are not surprised that they would evolve resistance, just as bacteria develop resistance to antibiotics.

But the CDC would like to preserve the benefits of Tamiflu and Relenza for as long as possible.

Tamiflu and Relenza not only fight flu. They can prevent infection if given soon enough. And a doctor at a North Carolina summer camp decided to protect 600 campers and staff there with so-called prophylactic doses of Tamiflu.

Two girls developed flu anyway. As they were cabin-mates, it is possible one infected the other, the CDC and North Carolina health investigators said.

Checks showed they were both infected with viruses that had mutations giving them resistance to Tamiflu.

So far, globally, just a handful of cases have been documented in which H1N1 swine flu resists the effects of Tamiflu. But world health officials are watching carefully.

The CDC recommends that Tamiflu and Relenza be saved to treat only people at risk of serious illness or death from flu - pregnant women, children who seem to have trouble breathing or other serious symptoms, and people with conditions that weaken their immune systems such as diabetes, asthma and heart disease.

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By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters)