Background: The US is in the midst of an epidemic of neurodevelopmentaldisorders (NDs). Thimerosal is an ethylmercury-containing compound added to some childhood vaccines.Several previous epidemiological studies conducted in the US have associated Thimerosal-containing vaccine(TCV) administration with NDs. Material/Methods: An ecological study was undertaken to evaluate NDs reportedto the Vaccine Adverse Event Reporting System (VAERS) from 1991 through 2004 by date of receipt and bydate of vaccine administration. The NDs examined included autism, mental retardation, and speech disorders.Statistical trend analysis was employed to evaluate the effects of removal of Thimerosal on the proportionof NDs reported to VAERS. Results: There was a peak in the proportion of ND reports received by VAERSin 2001-2002 and in the proportion of ND reports by date of vaccine administration in 1998. There weresignificant reductions in the proportion of NDs reported to VAERS as Thimerosal was begun to be removedfrom childhood vaccines in the US from mid-1999 onwards. Conclusions: The present study provides thefirst epidemiological evidence showing that as Thimerosal was removed from childhood vaccines, the numberof NDs has decreased in the US. The analysis techniques utilized attempted to minimize chance or bias/confounding.Additional research should be conducted to further evaluate the relationship between TCVs and NDs. Thisis especially true because the handling of vaccine safety data from the National Immunization Programof the CDC has been called into question by the Institute of Medicine of the National Academy of Sciencesin 2005.

Keywords: Child Development Disorders, Pervasive - epidemiology, Child, Preschool, Preservatives, Pharmaceutical - toxicity, Thimerosal - toxicity, United States, Vaccines - adverse effects