Background The influenza A (H1N1) 2009 monovalent vaccination programme was the largest mass vaccination initiative in recent US history. Commensurate with the size and scope of the vaccination programme, a project to monitor vaccine adverse events was undertaken, the most comprehensive safety surveillance agenda in the USA to date. The adverse event monitoring project identified an increased risk of Guillain-Barré syndrome after vaccination; however, some individual variability in results was noted. Guillain-Barré syndrome is a rare but serious health disorder in which a person's own immune system damages their nerve cells, causing muscle weakness, sometimes paralysis, and infrequently death. We did a meta-analysis of data from the adverse event monitoring project to ascertain whether influenza A (H1N1) 2009 monovalent inactivated vaccines used in the USA increased the risk of Guillain-Barré syndrome. Methods Data were obtained from six adverse event monitoring systems. About 23 million vaccinated people were included in the analysis. The primary analysis entailed calculation of incidence rate ratios and attributable risks of excess cases of Guillain-Barré syndrome per million vaccinations. We used a self-controlled risk-interval design. Findings Influenza A (H1N1) 2009 monovalent inactivated vaccines were associated with a small increased risk of Guillain-Barré syndrome (incidence rate ratio 2·35, 95% CI 1·42-4·01, p=0·0003). This finding translated to about 1·6 excess cases of Guillain-Barré syndrome per million people vaccinated. Interpretation The modest risk of Guillain-Barré syndrome attributed to vaccination is consistent with previous estimates of the disorder after seasonal influenza vaccination. A risk of this small magnitude would be difficult to capture during routine seasonal influenza vaccine programmes, which have extensive, but comparatively less, safety monitoring. In view of the morbidity and mortality caused by 2009 H1N1 influenza and the effectiveness of the vaccine, clinicians, policy makers, and those eligible for vaccination should be assured that the benefits of inactivated pandemic vaccines greatly outweigh the risks. Funding US Federal Government.
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