Characterization of the Increase in Narcolepsy Following the 2009 H1N1 Pandemic in Sweden

In the context of the H1N1 pandemic and the Pandemrix vaccination campaign, an increased number of narcolepsy cases were noted in several countries. In Sweden, this phenomenon was attributed to the effect of the Pandemrix vaccination in the first place. Interestingly, Denmark, a neighboring country, also had an increased incidence during this period, despite a low vaccination rate. Additionally, studies from China indicated that narcolepsy could occur as a consequence of the H1N1 infection itself. Against these background factors, and due to the clinical impression that narcolepsy patients who were vaccinated tend to dominate in the Swedish narcolepsy polyclinic, we performed an analysis of the increase with a specific interest in age and sex distribution. We also aimed to validate the origin of the excess cases, post hoc. Materials & Methods: The data for narcolepsy patients (ICD code G 47.4) distributed by sex and age at 5-year intervals, annually between 2005 and 2017, was retrieved from the National Patient Register. Information on the total population was collected from the Swedish Population Register and was used for calculations of the narcolepsy prevalence. Results: The number of narcolepsy cases increased markedly from 2009 to 2014 compared to the period before 2009. A particular increase in 2011 among children and teenagers was observed, which resulted in increased cases among the age group 20-39 years in the following years. The sex ratio did not change significantly during the study period. The excess cases found in our study exceeded the number of patients who so far (December 2023) received economic compensation for vaccine side effects (n=450). Conclusions: Our results confirm an association between the increased prevalence of narcolepsy cases and the Pandemrix vaccination. Consequently, the effect of the virus itself cannot be ruled out as a contributing factor. The delayed timing in diagnostics is crucial when determining the number of cases with onset during the pandemic.