The H1N1 pandemic virus can infect cells deeper in the lungs than those affected by seasonal flu viruses, scientists say.

Most people infected with swine flu have relatively mild symptoms. Doctors have also reported that some patients hospitalized with swine flu develop worse symptoms than seasonal flu victims.

'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 serious symptoms.'— Ten Feizi

The study in Wednesday's issue of the journal Nature Biotechnology confirms those anecdotal observations by doctors, as well as similar pathology reports from experiments on the lungs of ferrets, mice and nonhuman primates.

Seasonal influenza viruses attach to receptors found on cells in the nose, throat and upper airway, producing the telltale runny nose, scratchy throat and a dry cough.

In contrast, the H1N1 virus can also attach to receptors on cells deep inside the lungs, which can result in more severe lung infections, study author Prof. Ten Feizi of the Imperial College London and his colleagues found.

"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 serious symptoms," Feizi said in a release.

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

In laboratory experiments, the researchers studied 86 different receptors that were exposed to both seasonal H1 (A/Memphis/14/96-M) and the novel H1N1 pandemic flu virus.

The swine flu virus was able to latch on to the alpha 2-6 receptors in the upper respiratory tract as well as the alpha 2-3 receptors deeper in the lungs, although it attached more weakly lower down. The attachment to lung receptors resulted in more severe lung infection.