Immunizations against most vaccine-preventable diseases will be needed indefinitely unless the disease is eradicated. Public acceptance of immunizations may be threatened as vaccine coverage increases and disease decreases, however, due to the increase in both causally and coincidentally related vaccine adverse events. The post-marketing surveillance for such events in the USA in response to the mandatory reporting requirements of the National Childhood Injury Act of 1986. While VAERS has many methodological limitations intrinsic to such systems, it can play an important role in helping to monitor vaccine safety and maintain public confidence in immunizations.