The 14,543 spontaneous reports of suspected adverse reactions received in EudraVigilance from 1 November 2009 to 30 April 2010 for three centrally authorized Influenza A/H1N1 vaccines marketed in the European Economic Area (Celvapan, Focetria and Pandemrix) were extracted to evaluate the effectiveness of recommendations to strengthen pharmacovigilance systems during the pandemic and illustrate methods of signal detection used by the European Medicines Agency in this context. The number of vaccinees on 30 April 2010 was estimated to be at least 37,166,000 with a reporting rate of 391 per million vaccinees. 81.4% of reports were received in a period of 2 months ending 31 December 2009. Reports for A/H1N1 vaccines had fewer missing values for date of birth, age, case narrative, vaccination date and reaction onset date than reports involving human papilloma virus vaccines in a pre-pandemic period but more missing batch numbers (46.6%), with earlier notification by health care professionals to national authorities (median of 7 days since reaction onset date) and by national authorities to EudraVigilance (4 days). The network of European pharmacovigilance centers and the Agency was effective for monitoring the safety of A/H1N1 vaccines during the 2009-2010 influenza pandemic and coped with a sudden increase of the number of reports. Areas to be reinforced in order to improve the response to a future pandemic and to strengthen vaccine pharmacovigilance systems in general are highlighted. Observed-to-expected analyses were affected by uncertainties regarding the numbers of vaccinated individuals and age-specific background incidence rates. Imbalance analysis used by the Agency may overcome some of these limitations but needs further development. A multinational vaccine health outcome resource is needed to assess the burden of vaccine preventable diseases and the epidemiology of potential adverse outcomes, and to quickly evaluate safety signals, estimate the utilization, benefits and risks of vaccines and evaluate the effectiveness of public health measures.
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