A sudden increase in narcolepsy in Finnish children at the beginning of 2010 was likely related to the Pandemrix vaccine used in response to the H1N1 2009 flu pandemic, according to two reports published Mar. 28 in the open access journal PLoS ONE.

The authors of the studies, led by Markku Partinen of the Helsinki Sleep Clinic and Hanna Nohynek of the National Institute for Health and Welfare in Finland, found that the average annual incidence of narcolepsy between 2002 and 2009 among children younger than 17 was 0.31 per 100,000, and in 2010, this incidence was about 17 times higher, at 5.3 cases per 100,000. In contrast, the incidence rate for adults over 20 was essentially unchanged over that same time period.

To further evaluate the potential connection between the vaccine and narcolepsy, the researchers collected vaccination and childhood narcolepsy data for children born between January 1991 and December 2005.

They found that the narcolepsy incidence for vaccinated individuals within this age group was 9.0 per 100,000 people, as compared to just 0.7 per 100,000 for unvaccinated individuals – almost 13 times lower.

Together, these results provide compelling evidence that the Pandemrix vaccine, used in 2009 and 2010 in association with the H1N1 flu pandemic, contributed to narcolepsy in patients between the age of 4 and 19 in Finland, the authors conclude.