WASHINGTON - WITH H1N1 flu continuing to spread around the world, researchers say they have found the reason it is - so far - more a series of local blazes than a wide-raging wildfire.

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

But 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.

Even if it does not mutate, it is causing plenty of illness here and abroad already - and vaccine makers are working 'at full speed' to develop shots for use later this year if the government deems it enough of a threat, Dr Anthony Fauci, infectious disease director of the National Institutes of Health, said on Thursday.

The results of those tests will help the US government decide whether to distribute H1N1 flu vaccine starting in September, how much, and whether children or others should be first to get it.

The government wants public input before it makes any decisions, Dr Anne Schuchat of the Centres for Disease Control and Prevention told a meeting of the American Association for the Advancement of Science on Thursday.

It is currently flu season in the Southern Hemisphere, and viral spread in Argentina has prompted schools there to give students an early vacation. But H1N1 flu has not abated in the Northern Hemisphere, unusual since influenza usually retreats from summer's high heat and humidity.

Ram Sasisekharan, lead author of the report, meanwhile, warned that the H1N1 strain might just need a single change or mutation to make it resistant to Tamiflu.

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

The research was funded by the Singapore-MIT Alliance for Research and Technology and the National Institutes of General Medical Sciences. -- AP