A vaccine given to millions of children in Britain and Europe during the swine flu pandemic in 2009 and 2010 has been linked to narcolepsy in children, according to a new study by UK researchers.

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It is the first evidence from the UK showing a link between the H1N1 Pandemrix vaccine, made by British firm GSK, and narcolepsy. Previous studies in Finland and Sweden had demonstrated a link.

Researchers from Addenbrokes and Papworth hospitals in Cambridge looked at 75 children aged between four and 18 who were diagnosed with narcolepsy after January 2008. Eleven had been vaccinated with Pandemrix before showing signs of narcolepsy.

"Pandemrix may have triggered an immune reaction against the sleep centre cells in those children who were genetically predisposed to develop narcolepsy. This study has been important in helping to shed light on the mechanism of how narcolepsy can develop,” said Dr John Shneerson, one of the study authors.

The study published in the British Medical Journal found no link between seasonal flu vaccines, which are widely given every year in the UK. Though the authors warned other countries who administered their own pandemic vaccines may want to examine them for narcolepsy risk.

Brain malfunction

The disorder is linked to malfunction in a region of the brain which controls sleep patterns. Symptoms include daytime sleepiness and muscle weakness triggered by powerful emotions, often laughter - a condition called cataplexy.

The researchers concluded the risk of narcolepsy following vaccination was around one in every 55,000 doses of the vaccine. What they don't yet know is whether these children would have gone on to develop narcolepsy anyway. While the study shows a link between vaccination and the disorder, it hasn't shown a causal effect.

"These findings suggest there is an increased risk in children of narcolepsy after Pandemrix vaccination," said Professor Liz Miller, a consultant epidemiologist with the Health Protection Agency in London who led the study.

"However, this risk may be overestimated by more rapid referral of vaccinated cases. Long-term follow-up of people exposed to Pandemrix is needed before we can fully establish the extent of the association," she said.

'Pandemrix may have triggered an immune reaction against the sleep centre cells in those children who were genetically predisposed to develop narcolepsy' - Dr John Shneerson

By the end of 2009 more than 11 million doses of the vaccine had been distributed to doctors in the UK. It is estimated 30 million people across Europe were vaccinated with Pandemrix during the 2009/10 swine flu pandemic.

In a statement, GSK said it was conducting their own research into the link between narcolepsy and their vaccine. "We currently believe that the available data are insufficient to assess the likelihood of a causal association between Pandemrix and narcolepsy," it said.