No vaccine is perfectly safe. An adverse event can be said to be caused by a vaccine (i.e., a true reaction) if it is associated with a specific laboratory finding and a specific clinical syndrome or both. Alternatively, a clinical or epidemiological study is needed to find out whether the rate of a given syndrome in vaccinated individuals exceeds that expected among unvaccinated controls.

Vaccination may damage children in several ways. Live or attenuated virus vaccination can actually produce the infection that the vaccine is supposed to prevent.

A second mechanism of damage comes from neurotoxic materials found sometimes in vaccines. Thimerosol is the most widely discussed, since it contains mercury.

The third, and probably the most important theory of vaccine damage, relates to allergic reactions and the development of an autoimmune response, stimulated by the vaccine and its adjuvant. Vaccines always contain adjuvants, which are substances known to amplify the body’s response to the vaccine. These adjuvants are known to sometimes cause allergic and autoimmune responses on their own.