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Swine Flu May Be Less Dangerous Than Predicted: Study

Researchers challenge 'superbug theory' in animal tests using three flu strains

TUESDAY, Sept. 1 (HealthDay News) -- Fears that the H1N1 swine flu will turn into a "superbug" this year may be unfounded, say researchers at the University of Maryland.

In laboratory tests, the virus responsible for the swine flu pandemic did not take a virulent turn when combined with other strains of seasonal flu. But it did spread more rapidly than the other viruses, confirming the need for swine flu vaccinations, the researchers said.

The researchers exposed ferrets to three different viruses, the H1N1 swine flu and two seasonal strains of flu. The H1N1 strain dominated the others, reproducing by about twice as much, the researchers reported online in the journal PLoS Currents.

"The H1N1 pandemic virus has a clear biological advantage over the two main seasonal flu strains and all the makings of a virus fully adapted to humans," Daniel Perez, the lead researcher and program director of the University of Maryland-based Prevention and Control of Avian Influenza Coordinated Agricultural Project, said in a Sept. 1 university news release.

"I'm not surprised to find that the pandemic virus is more infectious, simply because it's new, so hosts haven't had a chance to build immunity yet. Meanwhile, the older strains encounter resistance from hosts' immunity to them," Perez added.

In the lab tests, after being infected with the new swine flu virus and one of the more familiar seasonal viruses (H3N2), some of the ferrets developed intestinal illness in addition to respiratory symptoms. The researchers hope further studies will determine if this type of co-infection and multiple symptoms are behind some of the deaths caused by the new pandemic virus.

Also, the swine flu virus caused infections deeper in the ferrets' respiratory system than the H1 and H3 seasonal viruses, which remained in the nasal passages.

"Our findings underscore the need for vaccinating against the pandemic flu virus this season," Perez said. "The findings of this study are preliminary, but the far greater communicability of the pandemic virus serves as a clearly blinking warning light."

Perez's team used samples of the H1N1 pandemic variety from last spring's initial outbreak of swine flu. They believe theirs is the first study to look at how swine flu interacts with seasonal flu viruses.

Another hopeful sign that the swine flu pandemic might be milder than predicted came last month from two infectious-disease experts at the U.S. National Institute of Allergy and Infectious Diseases.

Drs. David Morens and Jeffery Taubenberger challenged the notion that a mild flu in the spring can herald a more severe resurgence in the fall, a theory that has some scientists predicting a potentially dangerous swine flu resurgence this fall.

"Pandemic history suggests that changes neither in transmissibility nor in pathogenicity are inevitable," concluded Morens and Taubenberger in an article published in the Aug. 12 issue of the Journal of the American Medical Association.

Looking at the 1918-19 Spanish flu, which is thought to have killed 20 million to 40 million people worldwide, the researchers said the course of that illness varied greatly globally. They found no proof that it began in the spring with a less severe wave of infection and became more lethal through the summer as it picked up mutations.

They also studied 14 major flu epidemics dating to the 16th century and found no evidence supporting the "herald waves" theory.

Overall, "examination of past pandemics reveals a great diversity of severity," they said. "Some newer evidence [is] casting doubt on original herald wave theories."

More information

To learn about protecting yourself from swine flu, visit the U.S. National Institutes of Health.

-- Margaret Steele

SOURCE: University of Maryland, news release, Sept. 1, 2009; Journal of the American Medical Association, Aug. 12, 2009

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