They also say that all previous pandemic flu strains have had a specific genetic sequence in a gene called PB2 and the new H1N1 does not have this particular mutation.

Experts at the CDC say that H1N1 swine flu needs to be closely watched as any change may signal the virus is gaining the ability to spread more quickly and easily than it already does and may also mutate to become resistant to antiviral drugs; so far there have been two instances of this happening - one in Japan and one in Denmark.

The CDC team have also found that mutations allow the new H1N1 virus to live in the small intestine, which is something seasonal influenza cannot do, which may explain why so many swine flu patients have stomach upsets such as nausea and diarrhea.

Both teams of researchers confirm what doctors around the world are saying , that the new pandemic A(H1N1) influenza strain is slightly more pathogenic than seasonal influenza but nowhere near as dangerous as the 1918 pandemic virus or H5N1 avian influenza.

They disagree, however, on how easily the virus spreads with the Dutch team concluding that it does so very well, while the U.S. team believes that it's only moderately adept at jumping from one animal to the next or between humans.

Both studies are published in the journal Science.