Finnish researchers on Thursday have confirmed a direct link between the Pandemrix swine flu vaccine and the rise in narcolepsy cases in children.
Narcolepsy is a chronic nervous system disorder which causes people to become excessively drowsy, often uncontrollably falling asleep and, in more severe cases, suffering hallucinations or paralysing physical collapses called cataplexy.
In this age group, patients who were inoculated were 12.7 times more likely to develop narcolepsy than those who were not, although all of the patients who developed the disorder had a genetic predisposition to it, the study found.
The vaccine did not have an effect on the number of narcolepsy cases among children under four years of age or youth older than 19, according to the researchers.
In Finland, 79 children between the ages of four and nine developed narcolepsy after receiving the Pandemrix vaccine, which is a rate of six in 100,000.
Of these cases, an unusually high number, 76, also suffered from bouts of cataplexy, said THL.
Sweden, Norway, and France are also investigating the possible link between Pandemrix or the comparable vaccine Arepanrix and an increase in narcolepsy, but so far the link has only been shown in Finland and Sweden.
"The European Medicines Agency recommends that Pandemrix is used on those under 20 years of age only if seasonal influenza vaccines are not available, and only if the individual is at risk of developing severe influenza or other complications," said THL.
Finnish researchers announced in January that preliminary studies showed a link between Pandemrix and narcolepsy, but Thursday's announcement finalises this conclusion.
During the flu season of 2009-2010, laboratory tests confirmed 44 deaths from swine flu in Finland.
THL said that it agrees with the position of the European Medicines Agency that despite the "extremely unfortunate" cases of narcolepsy, the vaccine did more good than harm.
"Based on the data, we estimate that the swine flu vaccine prevented 40,000 infections in the 2009-2010 season and a further 40,000 infections in the 2010-2011 season," said THL.