PARIS: 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.