Change in risk for narcolepsy over time and impact of definition of onset date following vaccination with AS03 adjuvanted pandemic A/H1N1 influenza vaccine (Pandemrix) during the 2009 H1N1 influenza pandemic.

The magnitude of the estimated increased risk for narcolepsy following exposure to the A(H1N1) pandemic vaccine is highly dependent on the method used to determine the index date for disease onset. The sometimes very long and potentially variable interval from first symptom to a health care registry diagnosis complicates estimations of risk.