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LONDON (Reuters) - Children in Britain who were vaccinated with a GlaxoSmithKline shot against H1N1 swine flu had a significantly increased risk of developing the rare sleep disorder narcolepsy, according to results of a scientific study.
The findings, which have not yet been published in full, are the first firm evidence in Britain that the flu vaccine, called Pandemrix, is linked to narcolepsy cases in children.
Research in Finland, Sweden and Ireland has already found clear associations and Reuters reported last week that similar links were expected to be found in Britain.
Pandemrix was given to 30 million people - including around six million in Britain - during the 2009-2010 flu pandemic.
In a research summary, known as an abstract, seen by Reuters, the British researchers say "the increased risk of narcolepsy after Pandemrix suggests a causal association consistent with reports from Finland and Sweden."
The abstract shows that the research team, led by Liz Miller, a consultant epidemiologist at the UK Health Protection Agency (HPA), found an almost 10-fold increased risk in cases of the sleep disorder in children seen in sleep centres who had been immunised with Pandemrix.
A spokeswoman for the UK Health Protection Agency (HPA) could not confirm the 10-fold figure but issued a statement from Miller saying the results suggest an increased risk "consistent with findings from studies in other European countries."
Studies from Sweden, Finland and Ireland found the risk of developing narcolepsy was between seven and 13 times higher for children who had Pandemrix than for their unvaccinated peers.
Narcolepsy is a complex and life-long sleep disorder which is generally estimated to affect between 0.02 and 0.05 percent of the population, or between 200 and 500 people per million.
GSK, which has previously told Reuters its data show at least 795 people across Europe have reported developing narcolepsy linked to Pandemrix since the vaccine's use began in 2009, said on Thursday it was committed to finding out more.
"GSK initially became aware of possible cases of narcolepsy following vaccination with the adjuvanted H1N1 pandemic vaccine Pandemrix ...in 2010," it said.
It added: "We currently believe that the available data are insufficient to assess the likelihood of a causal association between Pandemrix and narcolepsy", but said it was committed to carrying out additional research.
The full HPA study with peer review is due to be published in the British Medical Journal in the coming months.
In total, Pandemrix was given to more than 30 million people in 47 countries during the 2009-2010 H1N1 swine flu pandemic. Because it contains an adjuvant, or booster, it was not used in the United States because drug regulators there are wary of adjuvanted vaccines.
Narcolepsy's most common symptoms are bouts of daytime sleepiness, but in its more severe forms it also brings nightmares, hallucinations, sleep paralysis and cataplexies - when strong emotions trigger a sudden loss of muscle strength.
It has no known cure and scientists are not clear what causes it, but specialist doctors say its symptoms can be treated with combinations of drugs aimed at re-regulating the patient's sleep-wake cycle.
Reporting by Kate Kelland; editing by Ralph Boulton