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The 2009 influenza A H1N1 pandemic placed unprecedented demand on public health authorities and the vaccine industry. Efforts were coordinated internationally to maximise the speed of vaccine development, distribution, and delivery, and the European Union's novel fast-track authorisation procedures mandated increased postmarketing surveillance to monitor vaccine safety. Clinicians in Finland and Sweden later identified an apparent increase in the incidence of narcolepsy associated with a specific adjuvanted pandemic influenza vaccine. After extensive review, the European Medicines Agency confirmed the existence of this association, which has since been detected in England, Ireland, France, and Norway. Assessments of the causal mechanisms continue. In this Review, we discuss how the narcolepsy association was detected, and we present the evidence according to the causality assessment criteria for adverse events following immunisation. The lessons learnt emphasise the central role of alert clinicians in reporting of suspected adverse reactions, and the importance of internationally robust postmarketing surveillance strategies as crucial components in future mass immunisation programmes.

Original publication




Journal article


The Lancet. Infectious diseases

Publication Date





227 - 238


Oxford Vaccine Group, Department of Paediatrics, Centre for Clinical Vaccinology and Tropical Medicine, University of Oxford, Oxford, UK; Oxford University Clinical Academic Graduate School, Medical Sciences Division, University of Oxford, Oxford, UK. Electronic address:


Humans, Narcolepsy, Influenza Vaccines, Vaccination, Incidence, Adverse Drug Reaction Reporting Systems, Influenza, Human, Influenza A Virus, H1N1 Subtype, Pandemics, Epidemiological Monitoring