Global Vaccine Safety


July 2003

Safety of smallpox vaccines

Two expert reports on the safety of smallpox vaccines were presented to the Global Advisory Committee on Vaccine Safety (GACVS) at its eighth meeting in Geneva, Switzerland on June 11-12, 2003. The reports included a detailed review of the safety of smallpox vaccine based on historical data as well as recent experience, particularly in the United States. 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.

Special attention was paid to the paucity of data regarding safety of immunization in subjects less than 18 years of age, age-related risks with the vaccine in general, and outcomes in women immunized during pregnancy. Current data are 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. 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 Committee concluded 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 potential risks to contacts of vaccinees, and that implementation of immunization would require 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.

Page last reviewed: 7 January 2009