In the late 1990s, some researchers started raising concerns over the amount of thimerosal -- a mercury-containing preservative -- found in many children's vaccines. Although thimerosal had been used as an anti-contamination agent for decades, until 1991 the diphtheria-tetanus-pertussis (DTaP) vaccination was the only thimerosal-containing shot recommended for infants and children. The hypothesis: As more thimerosal-containing vaccines like hepatitis B and Hib were added to the recommended schedule, researchers worried that babies were receiving too much of the chemical in too short a timeframe, which could potentially impact brain development.
In a totally separate (but coincidental) issue around this time, another group of researchers lead by a British doctor named Andrew Wakefield theorized that children who received the measles-mumps-rubella (MMR) vaccine (which never did contain thimerosal) were more likely to develop autism than those who did not receive it. By January 2011, however, Dr. Wakefield's study was discredited by the British Medical Journal.
Today, scientists are more confident than ever that vaccines play no role in the onset of this developmental disorder. Find out why here.