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

The researchers compared the way seasonal and pandemic H1N1 flu viruses infect cells by identifying which receptors each virus binds to. To do this, the researchers used a glass surface with 86 different receptors attached to it, called a carbohydrate microarray. When viruses were added to the glass surface, they stuck to their specific receptors and the corresponding areas on the plate 'lit up'. This meant the researchers could see which receptors the different viruses attached to.

Pandemic H1H1 influenza could bind strongly to receptors called ?±2-6, which are found in the nose, throat and upper airway, and it could also attach more weakly to ?±2-3 receptors, which are found on cells deeper inside the lungs. However, seasonal H1N1 influenza could only attach to ?±2-6.

"Receptor binding determines how well a virus spreads between cells and causes an infection," said Professor Feizi. "Our new study adds to our understanding of how swine-origin influenza H1N1 virus is behaving in the current pandemic, and shows us changes we need to look out for."