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In 1976, the vaccination campaign against influenza A (H1N1) virus was associated with an increase in Guillain-Barré syndrome (GBS; about 10 cases per 1 million vaccinations). Several surveillance systems have been monitoring the risk for adverse effects after 2009 H1N1 vaccination. This report reflects preliminary results from the CDC's Emerging Infection Program.
Between October 2009 and May 2010, 326 confirmed GBS cases were identified: 27 patients had documented 2009 H1N1 vaccination during the 42-day period before symptom onset, and 274 did not (the status of the remaining cases was unknown). An antecedent illness during the 42-day period before GBS onset was reported in 59% of vaccinated patients (and 78% of unvaccinated patients), pro…