Could H1N1 start to resist drugs?

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - The new H1N1 influenza virus is now widespread, causing a range of illness, and U.S. health officials said Tuesday they fear it could mix with drug-resistant versions of seasonal flu.

So far the new strain of swine flu, which has killed seven people in the United States and may have infected more than 100,000, responds well to treatment with antiviral drugs, officials at the U.S. Centers for Disease Control and Prevention say.

But this year its cousin, a seasonal H1N1 strain, became mostly resistant to the most commonly used antiviral -- Roche AG's Tamiflu. GlaxoSmithKline's Relenza, an inhaled drug, still works.

The CDC said more seasonal flu viruses were making people sick than would normally be expected for May, and the reasons were not clear.

"We think that it may be as much as half, or even more, of the viruses that we are testing now ... are the new H1N1 or cannot be subtyped," the CDC's Dr. Anne Schacht told reporters in a telephone briefing.

"The particular risk here ... is that co-circulation of this new virus together with the seasonal strains might put us at risk for there to be a reassortment event."

Reassortment is the viral equivalent of sex -- two viruses can meet and swap entire stretches of their genetic material. Flu viruses are especially prone to this and some pandemics have emerged because of this genetic mixing.

Flu viruses also mutate, by making mistakes when replicating.