Prior to marketing of pandemic influenza vaccines, the only safety data were those from clinical trials. The objective of this study was to compare information coming from spontaneous reporting with that systematically collected in a formal observation study; this also permits to further evaluate safety of pandemic influenza vaccines in the targeted patients' population. Out of a sample of 507 vaccinated subjects, 103 (20.3%) developed some complication. In the same period 83 reports corresponding to all vaccinated people of Castilla y León (n=131,462) were collected. Severe cases were 1 (1%) and 7 (8.4%), respectively, with the two procedures. The spontaneous reporting rate was 322-fold lower than that identified through the follow-up study; when considered the severe cases, it was 37-fold lower. Under certain circumstances reporting might be performing better than usual due to strengthening of the surveillance system. Adverse events observed for the pandemic H1N1 vaccines lie within the expected safety profile for common events with influenza vaccines. An overall benefit-risk assessment of these vaccines should be done.
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