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Immunization has played a huge role in better child health throughout the world. Every year, millions of deaths are prevented, and the risk of disability caused by infectious diseases is further reduced. Smallpox has been eradicated and polio is set, within the coming years, to be consigned to the history books.

No vaccine is however completely safe or provides 100% protection in all vaccinated individuals. Differences in the way individual immune systems react to a vaccine account for rare occasions when people are not protected following immunization or experience side-effects.

Although the vast majority of adverse events reported after vaccination are mild, some have been more serious. Some are due to the vaccine itself and constitute actual "vaccine reactions" such as those at the site of injection and some are linked to actual mistakes in vaccine preparation or administration error. Many events are, however, coincidental medical conditions. The reporting of such coincidental conditions can lead to undue fears and allegations.

Significant efforts have been made by regulatory authorities and manufacturers to further improve and ensure the safety of vaccines. Despite this, technology advances and increasing knowledge about vaccines have led to investigations focused on the safety of existing vaccines which have had an increasingly important effect on public confidence in immunization.

It is essential that concerns regarding vaccine-related adverse events are rapidly and effectively dealt with in order to maintain confidence in a vaccine, and ultimately maintain immunization coverage and reduce disease incidence. However, vaccine-associated adverse events can affect healthy individuals and should be promptly identified to allow additional research and appropriate action to take place.

It was in this context that WHO's Department of Vaccines and Biologicals, under the auspices of the Immunization Safety Priority Project, established, in 1999, the Global Advisory Committee on Vaccine Safety.

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