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Lancet. 2009 Dec 19;374(9707):2115-2122. doi: 10.1016/S0140-6736(09)61877-8. Epub 2009 Oct 31.

Importance of background rates of disease in assessment of vaccine safety during mass immunisation with pandemic H1N1 influenza vaccines.

Author information

Center for Global Health and Division of Infectious Diseases, Cincinnati Children's Hospital, Cincinnati, OH, USA. Electronic address:
National Institute for Health and Welfare, Helsinki, Finland.
Center for Vaccinology and Neonatal Immunology, Department of Pediatrics, University of Geneva, Geneva, Switzerland.
Institute for Vaccine Safety, Department of International Health, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA.
Division of Infectious Diseases, Department of Pediatrics, Dalhousie University, Halifax, NS, Canada.
Vaccine Safety Section, Centre for Immunization and Respiratory Infectious Diseases, Public Health Agency of Canada, Ottawa, ON, Canada.
Health Protection Agency, Centre for Infections, Colindale, London, UK.
National Vaccine Program Office, Department of Health and Human Services, Washington, DC, USA.
Center for Biologics Evaluation and Research, Food and Drug Administration, Rockville, MD, USA.
Discipline of Paediatrics, School of Paediatrics and Reproductive Health, University of Adelaide, SA, Australia.
Department of Pediatrics, Faculty of Medicine, University of São Paulo, São Paulo, Brazil.
Quality, Safety and Standards Team, World Health Organization, Geneva, Switzerland.
Immunization Safety Office, Centers for Disease Control and Prevention, Atlanta, GA, USA.

Erratum in

  • Lancet. 2010 Jan 30;375(9712):376.


Because of the advent of a new influenza A H1N1 strain, many countries have begun mass immunisation programmes. Awareness of the background rates of possible adverse events will be a crucial part of assessment of possible vaccine safety concerns and will help to separate legitimate safety concerns from events that are temporally associated with but not caused by vaccination. We identified background rates of selected medical events for several countries. Rates of disease events varied by age, sex, method of ascertainment, and geography. Highly visible health conditions, such as Guillain-Barré syndrome, spontaneous abortion, or even death, will occur in coincident temporal association with novel influenza vaccination. On the basis of the reviewed data, if a cohort of 10 million individuals was vaccinated in the UK, 21.5 cases of Guillain-Barré syndrome and 5.75 cases of sudden death would be expected to occur within 6 weeks of vaccination as coincident background cases. In female vaccinees in the USA, 86.3 cases of optic neuritis per 10 million population would be expected within 6 weeks of vaccination. 397 per 1 million vaccinated pregnant women would be predicted to have a spontaneous abortion within 1 day of vaccination.

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