Scientists think they can now explain a strange side-effect of the swine flu vaccine -- narcolepsy. It seems dozens of young people developed the rare disorder that makes them suddenly fall asleep after getting the shot.
Cases of narcolepsy have been reported among children and teenagers who received the H1N1 vaccine during the swine flu epidemic in 2009 in 12 countries, with Sweden and Finland reporting the most cases. 60 cases were reported in Finland.
According to the World Health Organization, scientists now say those people carry a gene that increases the risk for the disorder.
Finland's National Institute of Health and Welfare looked into all narcolepsy cases reported between 2006 and 2010, among people born after 1990. The analysis found an increased risk of narcolepsy among those ages 4 to 19 who received the Pandemerix H1N1 vaccine. The risk was about nine times higher among those who got the vaccine.
Of the 22 cases in Finland tested so far, all were carrying a gene known to increase the risk for the condition, the WHO announced on Tuesday.
"The National Institute considers it probable that the Pandemrix vaccine was a contributing factor to this observed increase, and has called for further investigation of other co-factors that may be associated with the increased risk," the WHO said. "They consider it most likely that the Pandemrix vaccine increased the risk of narcolepsy in a joint effect in those genetically disposed with some other, still unknown, genetic and/or environmental factor."
The WHO's Global Advisory Committee on Vaccine Safety "agrees that further investigation is warranted."
"An increased risk of narcolepsy has not been observed in association with the use of any vaccines, whether against influenza or other diseases in the past," it noted. "Even at this stage, it does not appear that narcolepsy following vaccination against pandemic influenza is a general worldwide phenomenon and this complicates interpretation of the findings in Finland."