1. Home>
2. Specialties>
3. Infectious Diseases>
4. Summary and Comment

Background Events and Mass Vaccination

Knowledge about background rates of disease events is needed to assess events temporally related to vaccination.

Events temporally related to vaccination are frequently attributed to the vaccine. Such concern may lead to program interruption, sometimes resulting in disease and death from vaccine-preventable infections. In anticipation of mass immunization against 2009 H1N1 influenza, researchers (some with industry support) identified disease events that might be attributed to the vaccines involved. Using available databases and published reports, they calculated the incidence of specific disease events (e.g., acute infectious and postinfectious polyneuritis [including Guillain-Barré syndrome], optic neuritis, new-onset multiple sclerosis, spontaneous abortion, sudden death, seizures, preterm labor) in various populations, including pregnant women.

The researchers found that the incidence of the disease events studied varied widely by age, sex, occupation, and location and that the variation was only partially explained by the type of reporting system. They then calculated the number of coincident events that might be expected in a population within 1 day, 1 week, and 6 weeks after receipt of a hypothetical vaccine.

The investigators predicted, based on background rates, that if 10 million individuals were vaccinated, about four cases of Guillain-Barré syndrome and one sudden death would occur within 1 week of vaccination as coincidental events. Similarly, for every 1 million pregnant women, 397 spontaneous abortions would be expected to occur within 1 day of vaccination, 2780 within 1 week, and 16,684 within 6 weeks, even if the vaccine does not increase risk.

Comment: In preparing for mass vaccination campaigns, scientists must identify background rates of events and use these rates to assess whether incidence has increased or a new adverse event has occurred, and whether an event is causally linked to vaccination. Education of health workers, the media, and others about expected coincidental events is essential. Editorialists describe multiple efforts to expand vaccine-safety monitoring systems, as well as international collaborations that will also provide timely data.

— Mary E. Wilson, MD

Published in Journal Watch Infectious Diseases November 10, 2009

Citation(s):

Black S et al. Importance of background rates of disease in assessment of vaccine safety during mass immunisation with pandemic H1N1 influenza vaccines. Lancet 2009 Oct 31; [e-pub ahead of print]. (http://dx.doi.org/10.1016/S0140-6736(09)61877-8)

DeStefano F and Tokars J. H1N1 vaccine safety monitoring: Beyond background rates. Lancet 2009 Oct 31; [e-pub ahead of print]. (http://dx.doi.org/10.1016/S0140-6736(09)61917-6)

2011 Top Stories