Negative stain electron microscopy image of the swine influenza. Credit: C. S. Goldsmith and A. Balish, CDC

Scientists studying the genome of the new swine flu virus say that it may not be as bad as first feared.

Public-health officials said today that they expect the virus to continue to spread around the country. However, so far, cases in the United States appear to be mostly mild, meaning that the ultimate public-health toll may be no worse than that of the typical seasonal flu, which kills an average of 36,000 people in the United States each year.

Early analysis of the genome seems to support that idea. According to an article in the Los Angeles Times,

"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.

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.

At a press conference today held by the Centers for Disease Control, acting director Richard Besser said that it's premature to say anything about the virulence compared with other strains of influenza based on genetic analysis.

It's also too soon to say what might happen over time. For example, viruses mutate constantly, so it's possible that this pathogen could become more deadly. "We are seeing slight changes as it moves through the community, but at this point we can't say if those changes impact how severe the virus is," said Besser at the press conference.

The 1918 virus began as a relatively mild flu, but then returned for a more deadly second wave. "If this virus keeps going through our summer, I would be very concerned," Palese told the Times.