A US health official has the new swine flu virus lacks genes that made the 1918 pandemic strain so deadly.
The Centres for Disease Control and Prevention said the new virus is a "very unusual" four-way combination of human genes and genes from swine 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".