Scientists see this flu strain as relatively mild

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.

Scientists have identified several other differences between the current virus and its 1918 predecessor, but the significance of those differences is still unclear, said Dr. Scott Layne, an epidemiologist at the UCLA School of Public Health.

Ralph Tripp, an influenza expert at the University of Georgia, said that his early analysis of the virus' protein-making instructions suggested that people exposed to the 1957 flu pandemic -- which killed up to 2 million people worldwide -- may have some immunity to the new strain.

That could explain why older people have been spared in Mexico, where the swine flu has been most 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."

"We expect to see more cases, more hospitalizations, and, unfortunately, we are likely to see more deaths from the outbreak," Health and Human Services Secretary Kathleen Sebelius told reporters Wednesday on her first day at work.

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%.

Dirk Brockmann, a professor of engineering and applied mathematics at Northwestern University in Evanston, Ill., used a computer model of human travel patterns to predict how this swine flu virus would spread in the worst-case scenario, in which nothing is done to contain the disease.