U.S. researchers believe they have found the reason why the swine flu virus has spread less effectively from person to person than other flu viruses.

The new H1N1 virus has a protein on its surface that is not very efficient at binding with receptors in people's respiratory tracts, researchers at the Harvard-MIT Division of Health Sciences and Technology report in Friday's edition of the journal Science.

"While the virus is able to bind human receptors, it clearly appears to be restricted," Ram Sasisekharan, lead author of the report, said in a news release on Thursday.

Flu viruses are known to mutate rapidly, the research team noted, so this one must be watched closely in case it changes to become easier to spread.

The discovery that the new virus can spread between people — but inefficiently — is consistent with the pattern of illness seen so far, said Sasisekharan. Most outbreaks have occurred in limited clusters, sometimes within a family or a school, but have not spread much further.

The researchers also noted that the new virus is more active in the gastrointestinal tract than the seasonal flu, leading to intestinal distress and vomiting in about 40 per cent of those infected.

Sasisekharan's paper, meanwhile, warned that the H1N1 strain might only need a single change or mutation to make it resistant to Tamiflu.

Vaccine makers, meanwhile, are working to develop shots for use in the fall and winter if the flu continues to spread.