* At least 14-fold increased narcolepsy risk after Pandemrix
* Boosted vaccine may have triggered adverse immune response
* GlaxoSmithKline says determined to understand more
* Drugmaker has reports of almost 800 cases across Europe
GlaxoSmithKline's Pandemrix swine flu vaccine has been linked to cases of the rare sleep disorder narcolepsy in children in a scientific study in England that confirms similar findings elsewhere in Europe.
The vaccine, more than 30 million doses of which were given during the H1N1 flu pandemic in 2009-2010, contains a booster, or adjuvant, and may have triggered an adverse immune reaction in some children at higher genetic risk of narcolepsy, scientists said in new research published on Wednesday.
Researchers at Britain's Health Protection Agency (HPA) who published the study in the British Medical Journal said the at least 14-fold increased risk they found had "implications for the future licensing and use of adjuvanted pandemic vaccines".
Narcolepsy is a life-long disorder and thought to be an autoimmune disease in which patient's immune system attacks the body's own cells. Its symptoms include frequent bouts of daytime sleepiness and in its severe forms it also causes night terrors, hallucinations and cataplexies - when strong emotions trigger a sudden loss of muscle strength.
Studies in Finland, Sweden and Ireland have also found a Pandemrix link to narcolepsy, and GSK says more than 800 cases linked to the shot have been reported in Europe.
A spokesman for the British drugmaker told Reuters on Wednesday: "We really want to get to the bottom of this and understand more about the potential role of Pandemrix in the development of narcolepsy."
He added, however, that GSK believes "the available data are insufficient to assess the likelihood of a causal association between Pandemrix and narcolepsy."
As Reuters reported earlier this month, scientists investigating the link further are homing in on the vaccine's adjuvant, a booster called AS03, and analysing whether its super-charging effect may have played a role.
According to the UK results, vaccination with Pandemrix at any time was associated with a 14-fold increased risk of narcolepsy, whereas vaccination within six months before onset of the disease was associated with a 16-fold increased risk.
"The increased risk of narcolepsy indicates a causal association," said the research team led by Liz Miller, a consultant epidemiologist with the HPA. They added, however, that because of variable delay in diagnosis, the risk may be overestimated because vaccinated children may have been referred to specialist sleep clinics more rapidly.
Scientists said the risk translated into around one in 50,000, lower than studies have found in other countries such as Finland and Sweden where Pandemrix was used more widely and the risk was around one in 16,000 to 17,000 children vaccinated.
In total, more than 30 million doses of the GSK shot were given in 47 mainly European countries during the H1N1 flu pandemic. It was not used in the United States.
The UK study looked at 75 children aged between four and 18 who were diagnosed with narcolepsy from January 2008 and who attended sleep centres across England. Eleven of the children had been vaccinated with Pandemrix before their symptoms began.
Finn stressed that Pandemrix is the only vaccine linked to this problem: "There is nothing to suggest that it occurs after other flu vaccines or vaccines against other diseases."
Narcolepsy is thought to be due to loss of function in cells called hypocretin cells in one of the brain's sleep centres.
John Shneerson, a consultant physician from the Respiratory Support and Sleep Centre at Papworth Hospital in Cambridge who co-led the UK study, said Pandemrix may have triggered an immune reaction against those cells, causing narcolepsy in some children who were genetically vulnerable.
Experts say around 25 percent of Europeans have a genetic profile making them more susceptible. Narcolepsy has no known cure, but specialist doctors say symptoms can be treated with drug combinations aimed at re-regulating the sleep-wake cycle.