Scientists see this flu strain as relatively mild

As the World Health Organization raised its infectious disease alert level Wednesday and health officials confirmed the first death linked to swine flu inside U.S. borders, scientists studying the virus are coming to the consensus that this hybrid strain of influenza — at least in its current form — isn’t shaping up to be as fatal as the strains that caused some previous pandemics….

Flu viruses are known to be notoriously unpredictable, and this strain could mutate at any point — becoming either more benign or dangerously severe. But mounting preliminary evidence from genetics labs, epidemiology models and simple mathematics suggests that the worst-case scenarios are likely to be avoided in the current outbreak.

"This virus doesn’t have anywhere near the capacity to kill like the 1918 virus," which claimed an estimated 50 million victims worldwide, said Richard Webby, a leading influenza virologist at St. Jude Children’s Research Hospital in Memphis, Tenn….

The Centers for Disease Control and Prevention and the National Institutes of Health published genetic sequence data Monday morning of flu samples isolated from patients in California and Texas, and thousands of scientists immediately began downloading the information. Comparisons to known killers — such as the 1918 strain and the highly lethal H5N1 avian virus — have since provided welcome news.

"There are certain characteristics, molecular signatures, which this virus lacks," said Peter Palese, a microbiologist and influenza expert at Mt. Sinai Medical Center in New York. In particular, the swine flu lacks an amino acid that appears to increase the number of virus particles in the lungs and make the disease more deadly….

The swine virus does appear able to spread easily among humans, which persuaded the WHO to boost its influenza pandemic alert level to phase 5, indicating that a worldwide outbreak of infection is very likely. And the CDC reported on its website that "a pattern of more severe illness associated with the virus may be emerging in the United States."…

But certainly nothing that would dwarf a typical flu season. In the U.S., between 5% and 20% of the population becomes ill and 36,000 people die — a mortality rate of between 0.24% and 0.96%…

Though scientists have begun to relax about the initial toll, they’re considerably less comfortable when taking into account the fall flu season. They remain haunted by the experience of 1918, when the relatively mild first wave of flu was followed several months later by a more aggressive wave.

The longer the virus survives, the more chances it has to mutate into a deadlier form…