The Centers for Disease Control and Prevention said the new virus is "a very unusual" four-way combination of genes from human, bird and pig viruses found in North America, Asia and Europe.
CDC flu chief Dr. Nancy Cox said the good news is "we do not see the markers for virulence that were seen in the 1918 virus." Nor does swine flu virus have the virulence traits found in the H5N1 strain of bird flu seen in recent years in Asia and other parts of the world, she said.
"However we know that there is a great deal that we do not understand about the virulence of the 1918 virus or other influenza viruses," that caused serious illnesses, she said. "So we are continuing to learn."
The relatively slow rate of spread this time around (as compared to the last three H1N1 pandemics for instance), and fairly low mortality, means that this time we may very well avoid the pandemic label. One possibility for this strain is that it will 'recirculate' for several years, much as SARS has. The one danger in that possibility is the availability for the virus to pick up new genetic codes, becoming a stronger more dangerous strain after one or two years of circulation. The really good news with such a scenario is that researchers have plenty of time to study for the virus, and assemble a powerful vaccine. The article concludes on an up-note, comparing virulence with this strain to seasonal flu and previous pandemics.
Another CDC official, Dr. Anne Schuchat, said preliminary studies suggest that in U.S. households with an infected person, about a quarter of other family members are getting sick as well. Generally, for seasonal flu, between 5 percent and 20 percent of those exposed to the virus get sick, depending on the setting.
In some pandemics, the rate has been as high as 35 percent, Cox said.
She noted the CDC has entered the gene information for the new virus into databases that are publicly available.