Scientists have discovered why people with asthma suffer severe attacks when they catch colds: their lungs do not produce enough of the anti-viral proteins needed to fight the infection. It is hoped that the finding will lead to a new generation of treatments that can prevent the most extreme asthma attacks.

One in 12 people in the UK suffers from asthma and around 1,500 die every year. The main causes of acute asthma attacks are common cold viruses. According to the charity Asthma UK, 60% of adults and 80% of children in hospital during an attack are suffering from a viral infection.

"When a normal person gets a common cold virus they get a common cold and a little bit of a cough but nothing else happening in the lungs, whereas an asthmatic may end up in hospital," said Sebastian Johnston, of Imperial College London, who led the research.

The difference is explained by a newly identified family of interferons, proteins produced in the lung by the immune system. "The interferons are directly protecting you against infection. They're deficient in people with asthma," said Professor Johnston. His results, which show that asthmatics produce fewer interferons compared with normal people, are published today in Nature Medicine.

Boosting the levels of interferons in the lungs of asthma sufferers would prevent the most severe attacks, something that current asthma treatments cannot do.

Prof Johnston said that there were often a few days between a cold starting and the first attack, giving people a chance to take the drugs. Trials on delivering interferon by inhalation have already begun.