In 1999, a study revealed that the preservative thimerosal, a mercury-containing compound present in many vaccines, caused several infants to have levels of mercury in their blood that exceeded guidelines recommended by the Environmental Protection Agency (EPA).

Preservatives are used in vaccines to reduce the risk of contamination by bacteria once the vial is opened. 

Exposure to high levels of mercury, especially in the developing child before birth, is associated with neurological disturbances. Therefore, parents began to fear that thimerosal may cause neurological difficulties, like autism.

When this study was first described, physicians, scientists, and public health officials quickly assessed the situation and found that: