Skip to main content

Neal Halsey Reaffirms Vaccines Do Not Cause Autism


Recently, there has been some confusion surrounding a hypothesis that the vaccine preservative thimerosal is linked to an increase in autism among children. Neal Halsey, MD, professor of International Health and director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health, does not and has not supported the belief that thimerosal or vaccines themselves cause autism in children, saying scientific evidence does not suggest any causal association between any vaccine and autism. Data published in the November 7, 2002, issue of the New England Journal of Medicine conclusively showed there was no difference in the rate of autism or related disorders in children who received measles, mumps, and rubella (MMR) vaccines compared to those who did not. Studies from other countries have determined that the hypothetical association between vaccines and autism was not supported by the available evidence.

An article published in the November 10, 2002, issue of New York Times Magazine included some statements attributed to Dr. Halsey that misrepresented his opinion, especially in the title and photo caption. A letter to the editor has been submitted, to correct the misunderstandings that have developed from this article.

Dr. Halsey does not believe that any vaccine increases the risk of autism and he did not say that thimerosal, which contains small amounts of ethyl mercury, represents a threat. Thimerosal is not a threat because it is no longer used as a preservative in vaccines administered to infants in the first 6 months of life*, when there is increased susceptibility of the developing brain to the effects of mercury and related compounds.

In 1999, Dr. Halsey became concerned that the use of thimerosal as a preservative in many vaccines led to some children being exposed to more ethyl mercury than was recommended based on guidelines from the Environmental Protection Agency for exposure to methyl mercury, a related product. Recent studies have determined that children whoas fetuses  were exposed to low to moderate amounts of methyl mercury through fish consumed by their mothers were at an increased risk for having mild neurological learning deficiencies. The findings from these studies did not show an association between methyl mercury exposure and autism. The Centers for Disease Control and Prevention (CDC) and the National Institutes of Health are supporting studies to determine if there is any definitive evidence of mild neurodevelopmental delay associated with the use of thimerosal as a preservative in vaccines. The preliminary results from one of two unpublished screening studies suggested the possibility of mild neurologic impairment associated with exposure to multiple doses of vaccines that contained thimerosal as a preservative, but there was no suggestion of an increased risk of autism. These studies require careful clinical evaluation of the exposed children before any definitive conclusions can be made.

As a precaution and in an effort to make vaccines as safe as possible, Dr. Halsey worked with the American Academy of Pediatrics and the Public Health Service in 1999 to urge reductions in exposure to mercury in all forms for infants and children and to discontinue using thimerosal as a preservative whenever possible. Now all vaccines routinely administered to infants and young children in the United States do not contain thimerosal as a preservative.

More information can be found on the Institute for Vaccine Safety website or from a review of this issue conducted by the Immunization Safety Review Committee of the Institute of Medicine.

* In 2005, the CDC and Advisory Committee on Immunization Practices recommended routine use of influenza vaccine in children 6-35 months of age. A special formulation of influenza vaccine, Fluzone, is available without thimerosal. More information about thimerosal content in vaccines can be found at

New York Times Correction 11/15/2002

Public Affairs Media Contacts for the Johns Hopkins Bloomberg School of Public Health:Tim Parsons or Kenna Brigham @ 410-955-6878 or