The researchers also found that the new H1N1 strain lacks genes that -- in other influenza A strains -- confer ease of transmission and virulence.
CDC officials discussed the findings at a teleconference Friday. The findings were released early in the journal Science because of the broad interest in this new strain of swine flu.
"From our analysis we have confirmed that the novel H1N1virus likely originated from pigs, based on data that each of the genetic components of this virus are most closely related to corresponding influenza virus genes identified from swine influenza viruses," said Dr. Nancy Cox, chief of CDC's Influenza Division.
However, this new virus is not similar to seasonal H1N1 viruses, she said. In their analysis of 70 samples of the new H1N1 virus from the United States and Mexico, the researchers found minor genetic differences, but consider the virus to be basically homogeneous, Cox noted.
Knowing the genetic make up of the virus makes it easier to come up with a candidate vaccine, Cox said. "We see much less variation among these new H1N1 viruses than we do for typical seasonal influenza viruses," she said.
Sequencing the virus' genetic code is also important for planning the public health response, including knowing which antiviral medications will be effective and which won't, Cox said.
And, Cox added, "We can take measures to be sure that the virus doesn't reemerge in a slightly different form."
In the future, scientists will need to keep a closer eye on pig populations to spot similar emerging flu viruses, the researchers said in the Science paper.
In the United States, most cases of the swine flu continue to be no worse than seasonal flu. Testing has found that the swine flu virus remains susceptible to two common antiviral drugs, Tamiflu and Relenza
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