The incidence of narcolepsy between January 2000 and December 2010 in children in western Sweden and its relation to the Pandemrix H1N1 influenza vaccination were assessed by collection of data from hospital and clinic medical records and by parent telephone interviews. Of 37 children identified with narcolepsy, 9 had onset of symptoms before the H1N1 vaccination and 28 had onset of symptoms within 12 weeks postvaccination. Median age at onset was 10 years. All patients in the postvaccination group were positive for human leukocyte antigen (HLA)-DQB1*0602. The incidence of narcolepsy was 25 times higher after the vaccination compared with the time period before. Pandemrix H1N1 vaccination is a precipitating factor for narcolepsy. Postvaccination narcolepsy has a lower age at onset and more sudden onset than generally seen. 
COMMENT. An abrupt increase in the incidence of childhood narcolepsy also followed an adjuvanted AH1N1 vaccine in Finland. The incidence of narcolepsy was 9.0 in the vaccinated as compared to 0.7/100,000 person years in unvaccinated individuals, the rate ratio being 12.7 . A similar increased risk of narcolepsy followed vaccination with adjuvanted pandemic A/H1N1 2009 vaccine in England . In contrast, no increase in narcolepsy diagnoses followed the H1N1 pandemic and vaccination campaign in Denmark, in South Korea , or in China . An autoimmune process is considered the most likely mechanism for the narcolepsy following influenza vaccination.