Influenza infection and Pandemrix ® vaccination and risk of relapse among multiple sclerosis patients: a nationwide population-based registry study in Norway

Introduction Influenza infection has been associated with various neurological complications. The cause of Multiple Sclerosis (MS) is not fully understood, but is thought to be immune-mediated. In MS patients, influenza infection may trigger the immune-response and possibly cause a relapse. The role of influenza infection is understudied, and safety of vaccination among MS patients is debated. We conducted a population-based study to determine the risk of a relapse among MS patients after an influenza infection and after pandemic vaccination. Methods The entire Norwegian population during 2008–2014 was defined as our study population ( n  = 5219,904). Data on exposures and outcomes from the Norwegian Patient Registry (NPR), the Norwegian Directorate of Health, the Norwegian Surveillance System for Communicable Diseases, the Norwegian Immunisation Registry and the Norwegian Prescription Database (NorPD) were linked using the unique 11-digit personal identification number. MS patients were defined, as either having at least one registry of MS in NPR and at least one MS medication dispensed or in cases with lack of information on medication from NorPD, at least two registries of MS from NPR was required. Relapse was defined as new contact with the health services and a new registration of International Classification of Diseases, Version 10 (ICD-10) code G35. Risk of relapse among MS patients was studied after an influenza infection (seasonal and pandemic influenza combined), pandemic influenza infection (influenza A [H1N1]), and vaccination with Pandemrix ® . Incidence rate ratios (IRRs) with 95% confidence interval (95% CI) in pre-defined risk periods compared with the background period were estimated in self-controlled case series. Results The IRR for relapse among MS patients within one week after an influenza infection (seasonal and pandemic influenza combined) was 8.14 (95% CI: 6.33–10.44). Considering pandemic influenza infection alone, similarly, risk of relapse among MS patients increased significantly within one week after an infection (IRR: 5.69 [95% CI: 3.02–10.71]). The IRR for other risk periods was not significant. Relapse among MS patients was not associated with the Pandemrix ® vaccination for any risk periods. Conclusions A significantly increased risk for relapse among MS patients was observed within the first week after an influenza infection (seasonal and pandemic influenza combined). The results indicated a similar trend also after pandemic influenza infection. Pandemrix ® vaccination was not associated with increased risk of relapse.