In 2009, an increased risk of narcolepsy (a chronic sleep disorder) was found following vaccination with Pandemrix, a monovalent 2009 H1N1 influenza vaccine that was used in several European countries during the H1N1 influenza pandemic. This risk was initially found in Finland, and then some other European countries also detected an association.

Pandemrix is manufactured by GlaxoSmithKline in Europe and was specifically produced for pandemic 2009 H1N1 influenza. Pandemrix was never licensed for use in the U.S.