Could H1N1 start to resist drugs?

WASHINGTON - The new H1N1 influenza virus is now widespread, causing a range of illness, and U.S. health officials said on 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 Schuchat 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.

The seasonal H1N1 that circulated in the United States this season was resistant to Tamiflu -- so resistant that the CDC recommended not even treating flu patients with the pills this year. "We would hate to see this novel strain essentially become resist through that mechanism," Schuchat said.

Tip of the iceberg
The CDC says there have been 5,469 confirmed cases of the new flu across most of the United States, but Schuchat has said this is only the tip of the iceberg. CDC acting director Dr. Richard Besser estimated last week there were 100,000 cases.

Officials in St. Louis County, Missouri, reported the seventh U.S. death on Tuesday — a 44-year-old man who had no underlying medical problems.

California state health officials released details of 30 of the early serious cases of H1N1 there, including a prematurely born baby who was infected in the hospital and several people with serious medical conditions.

"About two-thirds of the patients ... had at least one underlying condition that put them at risk," Schuchat said.

Five were pregnant women — something the CDC also cautioned about last week. Pregnancy can make a woman likely to have serious complications from any kind of influenza.

U.S. swine flu deaths hit double digits Britain criticized for efforts to halt spread Could H1N1 become resistant to drugs? MSN's swine flu coverage Discuss: How concerned are you?

Some are still in the hospital with very severe illness, Schuchat said. Although seasonal flu co-infections with bacteria often make for more serious disease, that was not the case in any of the H1N1 cases investigated, Schuchat said.

"We were surprised by the frequency of obesity among the severe cases that we are tracking," Schuchat said. It is not clear whether being obese makes a patient more vulnerable, or if the numbers reflect how common obesity in now in the U.S. population, she said.

Copyright 2009 Reuters. Click for restrictions.