Cases of Guillain-Barré syndrome (GBS) have been occasionally associated with influenza vaccines; this possible risk, even if rare, is a matter of much concern. To investigate the strength of this association, a systematic review and a meta-analysis have been conducted; for the purpose, controlled observational studies addressing the risk of GBS associated with different influenza vaccines were sought. We finally selected 39 studies of interest published between 1981 and 2014 (seasonal influenza vaccines, 22; pandemic influenza vaccines, 16; both vaccines simultaneously administered, 1); funnel plot did not identify publication bias. At the association between any influenza vaccine - whether seasonal or pandemic - with GBS, the overall relative risk was 1.41 (95% CI, 1.20-1.66). Pandemic vaccines presented a higher risk (RR = 1.84; 95% CI, 1.36-2.50) compared to seasonal vaccines (RR = 1.22; 95% CI, 1.01-1.48); the latter should be considered as marginally statistically significant. Pandemic adjuvanted vaccines were not found to be related to a higher risk compared to non-adjuvanted vaccines. The results of the present meta-analysis point to a small but statistically significant association between influenza vaccines, particularly the pandemic ones, and GBS, which is consistent with current explanations upon possible mechanisms for this condition to appear.
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