Still, it's too soon to draw any conclusions, said Nancy Cox, director of the CDC's influenza division.

CNN: CDC: U.S. may need 600 million swine flu vaccine doses

"The H7N9 situation is evolving very quickly, " said Nancy Cox, director of the Influenza Division at the U.S. Centers for Disease Control and Prevention.

CNN: New bird flu well-adapted to infect people

The new virus has genes from North American swine influenza, avian influenza, human influenza and a form of swine influenza normally found in Asia and Europe, said Nancy Cox, chief of the CDC's Influenza Division.

CNN: Among the swine flu mysteries: Why only deaths in Mexico?