WASHINGTON (AP) -- With swine 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-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 statement.

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.

On June 11, the World Health Organization declared a level 6 pandemic alert for H1N1. More than 300 people have died and more than 70,000 people have been infected, according to the WHO.

It's currently flu season in the Southern Hemisphere and the spread of the virus in Argentina has prompted schools there to give students an early vacation and one province to declare a public health emergency.

On Thursday, Health and Human Services Secretary Kathleen Sebelius said the United States will provide 420,000 treatment courses of the anti-viral medicine Tamiflu to the Pan-American Health Organization to help fight the flu in Latin America and the Caribbean.

"Viruses know no borders. The U.S. recognizes that a novel virus such as the H1N1 is a burden borne by all nations, and all of us have a responsibility to help support one another in the face of this challenge," Sebelius said at a meeting of health ministers in Mexico.

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

And the illness is not limited to the south.

England's health minister said Thursday that his country faces a projected 100,000 new swine flu cases a day by the end of August and must revamp its flu strategy. Britain has officially reported 7,447 swine flu cases and three deaths, but officials acknowledge the real number of cases is far higher, since many with the virus have not been tested.

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

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

They 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 percent of those infected.

The research by scientists from MIT and the federal Centers for Disease Control and Prevention was funded by the Singapore-MIT Alliance for Research and Technology and the National Institutes of General Medical Sciences.

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