Risks of neurological and immune-related diseases, including narcolepsy, after vaccination with Pandemrix: a population- and registry-based cohort study with over 2 years of follow-up.

Objectives To investigate the association between vaccination with Pandemrix and risk of selected neurological and immune-related diseases including narcolepsy. Design Population-based prospective cohort study using data from regional vaccination registries and national health registries. Setting Seven healthcare regions in Sweden comprising 61% of the Swedish population. Subjects Study population of 3 347 467 vaccinated and 2 497 572 nonvaccinated individuals (vaccination coverage ≈60%) followed between 2009 and 2011 for 6.9 million person-years after exposure and 6.0 million person-years without exposure. Main outcome measure and analysis First recorded diagnosis of neurological and immune-related diseases. Relative risks [hazard ratios ( HRs) with 95% confidence intervals ( CIs)] assessed using Cox regression, adjusted for covariates. Results For all selected neurological and immune-related outcomes under study, other than allergic vaccine reactions (for which we verified an expected increase in risk) and narcolepsy, HRs were close to 1.0 and always below 1.3. We observed a three-fold increased risk of a diagnosis of narcolepsy ( HR: 2.92, 95% CI: 1.78-4.79; that is, four additional cases per 100 000 person-years) in individuals ≤20 years of age at vaccination and a two-fold increase ( HR: 2.18, 95% CI: 1.00-4.75) amongst young adults between 21 and 30 years of age. The excess risk declined successively with increasing age at vaccination; no increase in risk was seen after 40 years of age. Conclusions For a large number of selected neurological and immune-related diseases, we could neither confirm any causal association with Pandemrix nor refute entirely a small excess risk. We confirmed an increased risk for a diagnosis of narcolepsy in individuals ≤20 years of age and observed a trend towards an increased risk also amongst young adults between 21 and 30 years. [ABSTRACT FROM AUTHOR]