The swine flu vaccine may have been responsible for a sudden increase in cases of narcolepsy among schoolchildren in Finland, a study has found.
The sleep disorder is characterised by periods of extreme drowsiness, sudden naps, and paralysis attacks.
Scientists spotted a sudden increase in narcolepsy rates among Finnish children early in 2010.
Research showed it was associated with the Pandemrix vaccine, given to children to protect them against H1N1 swine flu.
One study found that the incidence rate of narcolepsy among children and teenagers under the age of 17 shot up 17-fold after the vaccinations.
In contrast, the incidence rate for adults over 20 was unchanged.
The study compared narcolepsy rates between 2002 and 2009, and 2010.
During the first period, the rate for children was 0.31 per 100,000 individuals. After swine flu vaccination, it rose to 5.3 per 100,000.
A further study involving the same researchers, led by Dr Markku Partinen from the Helsinki Sleep Clinic, collected narcolepsy and vaccination data for children born between January 1991 and December 2005.
It found that narcolepsy incidence for vaccinated individuals was 13 times higher than for those who were unvaccinated.
Both studies appear in the online journal Public Library of Science ONE.
The researchers concluded: "We consider it likely that Pandemrix vaccination contributed, perhaps together with other environmental factors, to this increase in genetically susceptible children."
The scientists suspect the vaccine may have contributed to an auto-immune effect linked to narcolepsy.