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The World Health Organization is reviewing the safety of GlaxoSmithKline's Pandemrix H1N1 flu vaccine after a Finnish study suggested children who got the shot were nine times more likely to suffer from narcolepsy, a rare sleeping disorder.
Narcolepsy causes a person to fall asleep suddenly and unexpectedly. Its precise cause is unknown but it is generally considered to be triggered by a combination of genetic and environmental factors.
Researchers at Finland's National Institute for Health and Welfare (NIHW) said Tuesday their research suggested it was "most likely" the increase they found in narcolepsy was a joint effect of Pandemrix and some other factor or factors.
Their research, which was described as preliminary, was conducted by the Finnish national narcolepsy committee and published by the NIHW, found an increase in cases of narcolepsy among children aged four to 19 years who had the vaccine.
GSK said it was aware of the research but believed it was too soon to draw any conclusions. A separate investigation by European drugs regulators is already underway.
The Geneva-based WHO said in a statement that further investigation was required "concerning narcolepsy and Pandemrix vaccine" and it was working on this.
"WHO's Global Advisory Committee on Vaccine Safety (GACVS) is considering all available data relating to reports of increased rates of narcolepsy and is expected to issue a statement on its website within the coming days," it said.
Recommendations for the use of seasonal flu vaccines for 2010/2011 remain unchanged, it said, the issue of narcolepsy had not been linked with any other H1N1 pandemic flu vaccines, seasonal flu vaccines or any other adjuvanted, or boosted, vaccines used in childhood immunization programs.
GSK said it was reviewing the report and "it would be premature to draw any conclusions on a potential association between Pandemrix and narcolepsy until this European investigation has been completed."