http://www.parents.com/health/autism/vaccines/health-update-more-proof-that-vaccines-dont-cause-autism/?page=1
In the late 1990s, [1] 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, [2] researchers worried that babies were receiving too much of the chemical in too short a timeframe, which could potentially impact brain development.
To understand more about thimerosal safety, a brief chemistry/history lesson is in order. [3] Thimerosal was removed from most vaccines by 2001 because researchers worried that children were being exposed to too-high levels from receiving multiple vaccinations in a short timeframe.

[4] But this decision was based on what levels were considered safe for methyl mercury -- the kind in fish, which is structurally very different from the ethyl mercury found in thimerosal. Although scientists suspected that thimerosal was much safer than methyl mercury, they decided to remove it anyway, just to be super-careful.

[5] Now, new research published in the journal Pediatrics shows that babies excrete thimerosal too quickly for it to build up to dangerous amounts. In the study, researchers tested the blood mercury levels of Argentinean babies after they received routine childhood vaccinations (thimerosal is still used as a vaccine preservative there). They found that infants expel thimerosal about 10 times faster than fish mercury -- so rapidly that it can't accumulate in the body between vaccine doses.

"This study helps to debunk a crucial basis of the autism-vaccines theory, which held that babies were getting so many thimerosal-containing shots that the chemical would build up in the bloodstream and eventually cross over to the brain, where it could theoretically impact development," says study author Michael Pichichero, MD, a professor of microbiology/immunology and pediatrics at the University of Rochester Medical Center. "But thimerosal leaves babies' bodies way too quickly for that to happen, which just adds more proof that this theory is extremely unlikely."