PARIS (AFP) - Scientists on Sunday said they had figured out how influenza viruses carried by birds latch on to humans, a discovery that may open the way to a vaccine against not just deadly avian flu but against all flu types.

There are many strains of flu virus, but only a few have succeeded in crossing the species barrier from animals to humans.

Strains known as H1 and H3 are the most common, and are especially efficient in attacking cells in the upper reaches of the respiratory system. Variants of the H5 virus, by contrast, usually remain confined to wild or domesticated fowl.

But when they do infect humans it is often with lethal results, as immune systems are unable to recognise and counter the novel pathogen.

Of 348 confirmed cases of H5N1 avian flu in the last five years, 216 -- more than 60 percent -- have died as a result, according to the World Health Organisation (WHO).

What health officials fear most is the emergence of a new H5 strain that can easily "jump" from birds to humans, potentially unleashing a pandemic on the scale of the "Spanish flu" of 1918-19 that killed tens of millions of people.

The findings, published in the British journal Nature, overhaul scientific understanding of how viruses attach themselves to cells inside human lungs.

Researchers have long known that whether an influenza strain infects humans depends on the ability of a protein on the surface of the virus, called hemagglutinin, to bind to a sugar receptor in the respiratory tract.

In humans, these receptors are known as alpha 2-6, whereas their counterparts in birds are known as alpha 2-3.

Up to now, scientists believed it was a genetic switch in the virus that allowed it to bind to human rather than bird receptors, thus making the much-feared "species jump" possible.

But the study, led by Massachusetts Institute of Technology (MIT) professor Ram Sasisekharan, says that the big factor is the shape of the sugar receptors in human lung cells.

The human alpha 2-6 receptors come in two shapes, one broadly resembling an umbrella, and the other a cone. To infect humans, flu viruses must bind to the umbrella-shaped receptors, the researchers found.

"This work enables researchers to look at flu viruses in an entirely new way," said Jeremy Berg, director of the National Institute for General Medical Sciences, which funded the research.

At the very least, the new discovery will help scientists rapidly identify strains that may develop the capacity to attack human respiratory systems.

"Now that we know what we are looking for, this could help us not only monitor the bird flu virus, but it can aid in the development of potentially improved therapeutic interventions for both avian and seasonal flu," said Sasisekharan.

Some 500,000 people around the world die every year from seasonal influenza, in which a strain mutates slightly from previous strains.

A virus that would cause a pandemic, though, would be genetically so new that immune systems and vaccines would not be primed to recognise it. The "Spanish flu" virus killed as many as 50 million people, although the toll is widely disputed.