Monitoring the safety of vaccines: assessing the risks.

The safety of vaccines, particularly the most widely used vaccines to which most children are exposed as infants and toddlers, has always been an extremely high priority for vaccine manufacturers and government agencies. Products intended for healthy people must be held to a high standard of safety assurance. In addition to the intense safety assessments conducted prior to licensure, post-marketing surveillance programmes are essential to identify and study possible risks that occur too rarely to have been identified in pre-licensure studies or that occur in populations not studied in pre-licensure studies. Studying rare risks of vaccines is more complex than for therapeutic products because the exposure is virtually universal for many vaccines, ensuring occurrence simply by chance of many adverse outcomes in temporal association with vaccination. In the US the Vaccine Safety Datalink (VSD), a consortium of managed care organisations, has been established to study more rigourously possible vaccine-associated risks. These risks may be identified through reports to the Vaccine Adverse Event Reporting System (VAERS), the nationwide passive surveillance programme, as well as other sources. The combination of passive surveillance and more structured case-control or cohort studies possible in the VSD has helped to both identify new vaccine risks and to provide reassuring evidence of lack of risk in other situations where concerns have been raised.