The paper provides the first laboratory corroboration of reports from front-line doctors that some patients with A(H1N1) virus suffer worse symptoms compared to those with run-of-the-mill seasonal flu.

Influenza viruses penetrate cells by attaching themselves to molecules called receptors, located on the outside of the cell wall.

The virus' docking spike and the receptor are like a key and a lock. Some viruses only open a few cells, while others have something rather closer to a pass key, making it possible to infect a larger cell variety.

Once inside, the virus uses the cell's machinery to replicate, eventually bursting the cell and going on to attack others.

Seasonal strains attach themselves almost exclusively to cells found in the nose, throat and upper airway, producing a some of influenza's signature symptoms: runny nose, scratchy throat, a dry cough.

But the new research, published in the journal Nature Biotechnology, shows that the new swine flu -- by sticking to a greater range of receptors -- can also breach cells deep in the lungs.

Lab-dish experiments were carried out by Ten Feizi of Imperial College London and colleagues in which 86 different receptors were exposed to both seasonal and pandemic flu.

The seasonal strains only locked onto the kind of receptors found in the upper respiratory track.

But the swine flu virus was also able to latch onto receptors found deep inside the lungs, albeit more weakly. The adhesion results in a more severe lung infection.

Feizi spelt out the risk of what could happen if natural selection encourages viral strains that favour this deep-lung penetration.

"If the flu virus mutates in the future, it may attach to the receptors deep inside the lungs more strongly, and this could mean that more people would experience severe symptoms," said Feizi.

"We think scientists should be on the lookout for these kinds of changes in the virus so we can try to find ways to minimise the impact."

Using a statistical technique, epidemiologists sifting through data from other countries have found similarly disquieting patterns.

French epidemiologist Antoine Flahault has reported a 100-fold increase, compared to seasonal flu, in the number of swine flu deaths in Mauritius and New Caledonia attributed directly to the virus itself rather than secondary bacterial infections or underlying conditions.

Many of those deaths were caused by acute respiratory disease syndrome (ARDS), which requires intensive-care treatment for an average of three weeks. Only 50 percent of ARDS patients survive.

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