The new H1N1 virus appears to outcompete seasonal flu, making it less likely to mix with other circulating flu viruses into a "superbug" as some had feared, US researchers said.

The H1N1, or swine flu, virus also spreads more quickly and causes more severe disease in animal studies, the team said, but it shows no signs of mixing with either of the two seasonal flu viruses to form a new, so-called reassortant virus.

"The results suggest that 2009 H1N1 influenza may outcompete seasonal flu virus strains and may be more communicable as well," said Dr. Anthony Fauci, of the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health.

The study, conducted in ferrets by a team at the University of Maryland, underscores the need for getting vaccinated with the new swine flu vaccine, Fauci said in a statement.

When the team infected ferrets with 2009 H1N1 virus plus either seasonal H1N1 virus or seasonal H3N2 virus, both viruses made them sick, but only the H1N1 virus spread to other ferrets, suggesting it will dominate ordinary flu.

"The H1N1 pandemic virus has a clear biological advantage over the two main seasonal flu strains and all the makings of a virus fully adapted to humans," Daniel Perez of the University of Maryland said in a statement.

"I'm not surprised to find that the pandemic virus is more infectious, simply because it's new, so hosts haven't had a chance to build immunity yet. Meanwhile, the older strains encounter resistance from hosts' immunity to them," Perez said.

The researchers also found that the pandemic virus established infections deeper in the ferrets' respiratory system, including the lungs, while the seasonal flu strains remained in the nasal passages.