Global Vaccine Safety

Statement from the Global Advisory Committee on Vaccine Safety (January 2004)

An updated account of the safety of smallpox vaccination, was presented to the Global Advisory Committee on Vaccine Safety (GACVS) at its ninth meeting in Geneva, Switzerland on December 3-4, 2003. Two expert reports on the safety of smallpox vaccination were previously presented to GACVS, including detailed reviews of historical data and of recent experience, particularly in the United States.1 The data highlighted the large variation in the pathogenicity of previously used strains and it was noted that safety data gained with the older vaccines may not necessarily apply to newly developed smallpox vaccines and it should not be assumed that they will be safer.

The Committee concluded the data were insufficient to define the incidence of adverse events in primary vaccinees as opposed to individuals re-vaccinated after a long interval. The Committee noted the importance of adverse event surveillance programs being open-minded so that hitherto unrecognized events might be detected. The Committee further noted that if the vaccine is being used in mass campaigns, it would be especially important for smallpox immunization programmes to be supported by adverse event monitoring. Implementation in settings where there is no educational programme and careful exclusion of volunteers at potentially increased relative risks (e.g., those with HIV) could possibly result in enhanced vaccine risks. The risk of exposure should be carefully considered in relation to each specific scenario that may arise. In mass vaccination, the impact of vaccine reactions may constitute a significant health burden.

The current update was based on 38,759 persons vaccinated in the United States since January 2003, covering 65% of health care workers and at least one health care worker in 45% of hospitals. There is, in addition, a pregnancy registry of 160 women exposed to smallpox vaccine during or immediately before pregnancy, identifiable by testing; data from the pregnancy registry are pending. The Committee noted that consistent adverse effects reported in smallpox vaccinees have been myopericarditis and dilated cardiomyopathy (both not previously well recognized as serious adverse events to the smallpox vaccine); the frequency of each adverse effect exceeds what might have occurred by coincidence.

The Committee did not change the conclusions from its June 2003 meeting1 that there is a real risk of serious adverse events following immunization with smallpox vaccine, including safety issues that have not previously been recognized, that there may be significant potential risks to contacts of vaccinees, and that implementation of immunization would require significant capacity and resources. The Committee will continue to monitor the safety of smallpox vaccines.

The GACVS is a scientific advisory body established by WHO to provide a reliable and independent scientific assessment of vaccine safety issues in order to respond promptly, efficiently and with scientific rigour to such issues. Membership includes experts from around the world in the fields of epidemiology, paediatrics, internal medicine, pharmacology and toxicology, infectious diseases, public health, immunology and autoimmunity, drug regulation, and safety.


1WER No. 32, 2003, pp. 282–284.

Last page revised: 7 January 2009

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