The risk of Guillain-Barré syndrome associated with influenza A (H1N1) 2009 monovalent vaccine and 2009-2010 seasonal influenza vaccines: results from self-controlled analyses.

Purpose: The Centers for Disease Control and Prevention Emerging Infections Program implemented active, population-based surveillance for Guillain-Barré syndrome (GBS) following H1N1 vaccines in 10 states/metropolitan areas. We report additional analyses of these data using self-controlled methods, which avoid potential confounding from person-level factors and co-morbidities.

Methods: Surveillance officers identified GBS cases with symptom onset during October 2009-April 2010 and ascertained receipt of H1N1 vaccines. We calculated self-controlled relative risks by comparing the number of cases with onset during a risk interval 1-42 days after vaccination with cases with onset during fixed (days 43-84) or variable (days 43-end of study period) control intervals. We calculated attributable risks by applying statistically significant relative risks to an independent estimate of GBS incidence.

Results: Fifty-nine GBS cases received H1N1 vaccine with or without seasonal vaccine. The relative risk was 2.1 (95%CI 1.2, 3.5) by the variable-window and 3.0 (95%CI 1.4, 6.4) by the fixed-window analyses. The corresponding attributable risks per million doses administered were 1.5 (95%CI 0.3, 3.4) and 2.8 (95%CI 0.6, 7.4).

Conclusions: These attributable risks are similar to those of some previous formulations of seasonal influenza vaccine (about one to two cases per million doses administered), suggesting a low risk of GBS following the H1N1 vaccine that is not clearly higher than that of seasonal influenza vaccines.