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New CDC study on the effect of thimerosal exposure on neuropsychological characteristics of children is fueling the debate about the origin of autism


Thimerosal (also known as Thiomersal) is an organic compound that contains ethyl mercury. It has been used in vaccines since the 1930s to prevent bacterial and fungal contamination. Since organomercury compounds are strong neurotoxins, some parents’ groups and prominent legislators contend that thimerosal has caused an epidemic of childhood autism. In 2001, the Institute of Medicine determined that such a relationship is biologically plausible, but that not enough evidence exists to support or reject this hypothesis.

Autism Spectrum Disorders are a group of developmental disabilities that are caused by an abnormality in the brain.  According to the Centers for Disease Control and Prevention (CDC), 1 in 500 (2/1,000) to 1 in 150 children (6.7/1,000) have an Autism Spectrum Disorder (ASD) and four boys are diagnosed to every one girl. ASDs range from a severe form, autistic disorder (classic autism) to a mild form, Asperger syndrome. If a child has symptoms of either disorder but does not meet the specific criteria of either, he/she is diagnosed with pervasive developmental disorder not otherwise specified (PPD-NOS). Autism was unknown until 1943, when it was identified and diagnosed among eleven children born in the months after thimerosal was first added to baby vaccines in 1931. However, several epidemiologic studies have examined this question and found no evidence that thimerosal is associated with autism. In 2002 thimerosal was eliminated from many childhood vaccines, however there has been no evidence that autism is on the decline since then.

The new study:
Researchers with the Centers for Disease Control (CDC) and colleagues gave performance tests to 1,047 children ages 7 to 10 for whom they had lifetime vaccination records and tracked how increasing exposure to thimerosal correlated with neuropsychological outcomes, such as IQ, fine motor skills and language.

Five percent, or 19 of 378 statistical tests scores analyzed, showed significant results, which is the number that can be predicted to result from chance alone, says Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases at the CDC.

The significant links went in both directions. Twelve indicated thimerosal exposure improved test performance, while seven showed it resulted in poorer performance.

The lone worrisome result was that, boys who had highest thimerosal exposure between birth and 7 months, showed a twofold greater risk of verbal and motor tics, which are involuntary movements or sounds. At least one earlier study had found a similar association.

Dr. Jeffrey Baker, a pediatrician and vaccine expert who is director of the history of medicine program at Duke University, said that the study’s findings should be reassuring for parents. “This study will further strengthen a growing consensus among researchers that there is no real evidence that thimerosal in vaccines led to any actual harm,” Dr. Baker said.

But Sallie Bernard, executive director of SafeMinds, a nonprofit parent organization whose members contend that thimerosal injured their children, said the study was inconclusive.

The study is unlikely to end the increasingly charged debate about vaccine safety. Indeed
doubts are fueled by some shortcomings of the study:
  • While the data showed that higher exposure to mercury was associated with significantly poorer performance on tests of backward recall, behavioral regulation problems and a higher likelihood of motor and phonic tics, the study concludes that no relationship was found.
  • The study investigated only 1047 participants. A very small group for studying the loading on 378 tests. (Remember, autism is observed in 2-6 out of 1000 children !)
  • A very low (30%) recruitment rate, which did not allow to adequately control for potential selection bias.
  • Limited data variation
  • The study did not assess thimerosal’s association with autism directly. The Centers for Disease Control and Prevention is conducting a separate autism and thimerosal study that is expected to be published next year.
  • Four of the doctors who led the study have consulting relationships with vaccine makers who could lose billions from a scandal.
  • The researchers didn't adjust for birth weight -- "which doesn't make sense, given that an 8 pound baby injected with the hepatitis B vaccine at birth was exposed to 35 times the EPA daily safety level for mercury, (calculated by bodyweight) while a 4 pound infant was slammed with 70 times the EPA level."
  • The researcher did not control for physical, behavioral, or pharmaceutical therapies: the kids were enrolled in a big California HMO, so they could have received treatments that minimized thimerosal's effects.
  • Children with autism were excluded from this study. In fact, the study specifically did not look at children with autism as the sample size was too small and the testing is impossible to complete for the typical child with autism. The exclusion of children with autism from the study may have undermined the power of the study to draw any conclusions about thimerosal.
There is a considerable discrepancy between toxicological studies clearly showing the neurotoxicity of thimerosal and epidemiological studies, failing to find any relationship between thimerosal exposure and neurological damage. The prudent and scientifically responsible thing to do is to evaluate multiple lines of evidence and look at the totality of the data before drawing any conclusion, especially when it comes to something as complex a scientific challenge as this.

The Original study

W.W. Thompson, C. Price, B. Goodson, D.K. Shay, P. Benson, V.L. Hinrichsen, E. Lewis, E. Eriksen, P. Ray, S.M. Marcy, J. Dunn, L.A. Jackson, T.A. Lieu, S. Black, G. Stewart, E.S. Weintraub, R.L. Davis, F. DeStefano, Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years, N. Engl. J. Med., 357/13, 2007, 1281-1292. DOI: 10.1056/NEJMoa071434

Related studies

  David A. Geier, Mark R. Geier, Early Downward Trends in Neurodevelopmental Disorders Following Removal of Thimerosal-Containing Vaccines, J. Am. Phys. Surg., 11/1  (2006) 8-13. fulltext available at: http://www.jpands.org/vol11no1/geier.pdf

D.A. Geier, M.R. Geier, An evaluation of the effects of thimerosal on neurodevelopmental disorders reported following DTP and Hib vaccines in comparison to DTPH vaccine in the United States, J. Toxicol. Environ. Health Part A, 69/16 (2006) 1481-1495. DOI: 10.1080/15287390500364556

Eric Fombonne, Rita Zakarian, Andrew Bennett, Linyan Meng, Diane McLean-Heywood, Pervasive Developmental Disorders in Montreal, Quebec, Canada: Prevalence and Links With Immunizations, Pediatrics, 118 (2006) e139-e150. DOI: 10.1542/peds.2005-2993

S.J. James, William Slikker III, Stepan Melnyk, Elizabeth New, Marta Pogribna, Stefanie Jernigan, Thimerosal Neurotoxicity is Associated with Glutathione Depletion: Protection with Glutathione Precursors, Neurotoxicol., 26 (2005) 1-8. DOI: 10.1016/j.neuro.2004.07.012

Sarah K. Parker, Benjamin Schwartz, James Todd, Larry K. Pickering, Thimerosal-Containing Vaccines and Autistic Spectrum Disorder: A Critical Review of Published Original Data, Pediatrics, 114 (2004) 793-804. DOI: 10.1542/peds.2004-0434

Thomas M. Burbacher, Danny D. Shen, Noelle Liberato, Kimberly S. Grant, Elsa Cernichiari, Thomas Clarkson, Comparison of Blood and Brain Mercury Levels in Infant Monkeys Exposed to Methylmercury or Vaccines Containing Thimerosal, Environ. Health Perspect., 113 (2005) 1015-1021. DOI: 10.1289/ehp.7712

P. Offit, J. Golden, Thimerosal and Autism, Mol. Psychiatry, 9 (2004) 644-645. DOI: 10.1038/sj.mp.4001522

D. Bernard, Association between thimerosal-containing vaccine and autism, J. Am. Med. Assoc., 291/2 (2004) 180-181. DOI: 10.1001/jama.291.2.180-b

D. Geier, M.R. Geier, Neurodevelopmental disorders following thimerosal-containing childhood immunizations: a follow-up analysis, Int. J. Toxicol., 23/6 (2004) 369-376. DOI: 10.1080/10915810490902038

Mark F. Blaxill, Lyn Redwood, Sallie Bernard, Thimerosal and autism? A plausible hypothesis that should not be dismissed, Med. Hypotheses, 62 (2004) 788-794. DOI:10.1016/j.mehy.2003.11.033

M.R. Geier, D.A. Geier, Neurodevelopmental disorders following thimerosal-containing vaccines, Exp. Biol. Med., 228 (2003) 660-664. DOI: 10.1177/153537020322800603

L. Magos, Neurotoxic character of thimerosal and the allometric extrapolation of adult clearance half-time to infants, J. Appl. Toxicol., 23 (2003) 263-269. DOI: 10.1002/jat.918

Anders Hviid, Michael Stellfeld, Jan Wohlfahrt,  Mads Melbye, Association Between Thimerosal-Containing Vaccine and Autism, J. Am. Med. Assoc., 290 (2003) 1763-1766. DOI: 10.1001/jama.290.13.1763

K.B. Nelson, M.L. Bauman, Thimerosal and autism ?, Pediatrics, 111 (2003) 674-679. DOI: 10.1542/peds.111.3.674

M.R. Geier, D.A. Geier, Response to critics on the adverse effects of thimerosal in childhood vaccines, J. Am. Phys. Surg., 8/3 (2003) 68-70.

M.R. Geier, D.A. Geier, Thimerosal in childhood vaccines, neurodevelopment disorders, and heart diseases in the United States, J. Am. Phys. Surg., 8/1 (2003) 6-11.

K.M. Madsen, M.B. Lauritsen, C.B. Pedersen, P. Thorsen, A.-M. Plesner, P.H. Andersen, P.B. Mortensen, Thimerosal and the Occurrence of Autism: Negative Ecological Evidence From Danish Population-Based Data,  Pediatrics, 112/3 (2003) 604-606. DOI: 10.1542/peds.112.3.604

Thomas Verstraeten, Robert L. Davis, Frank DeStefano, Tracy A. Lieu, Philip H. Rhodes, Steven B. Black, Henry Shinefield, Robert T. Chen, Safety of Thimerosal-Containing Vaccines: A Two-Phased Study of Computerized Health Maintenance Organization Databases, Pediatrics, 112/5 (2003) 1039-1048. fulltext available at:  http://pediatrics.aappublications.org/cgi/reprint/112/5/1039

M.E. Pichichero, E. Cernichiari, J. Lopreiato, J. Treanor, Mercury concentrations and metabolism in infants receiving vaccines containing thimerosal: a descriptive study, Lancet, 360 (2002) 1737-1741. DOI: 10.1016/S0140-6736(02)11682-5

L.K. Ball, R. Ball, R.D. Pratt,  An assessment of thimerosal use in childhood vaccines, Pediatrics, 107/5 (2001) 1147-1154. DOI: 10.1542/peds.107.5.1147

Organizations related to Autism

National Autism Association (NAA)
SafeMinds: Sensible Action For Ending Mercury-Induced Neurological Disorders
Generation Rescue
FAIR: Foundation for Autism Information and Research, Inc.
Put Children First
Autism speaks
IAN: Interactive autism network
A-CHAMP: Advocates for Children' Health Affected by Mercury Poisoning

Related information

Wikipedia: Thiomersal controversy
U.S. Food and Drug Administration: Information about Thimerosal in Vaccines
Centers for Disease Control and Prevention (CDC): Frequently asked questions about Thimerosal
NIAID Research on Thimerosal
Toxic Exposure Study Trust (TEST) Foundation: Toxicity of Thimerosal
EVISA Link Database: Thimerosal
Board on Health Promotion and Disease Prevention, Institute of Medicine. 2004. Immunization Safety Review: Vaccines and Autism. Washington, D.C.: National Academies Press. Available at http://www.nap.edu/books/030909237X/html/.
Prisonplanet.com, September 28, 2007: How Mercury Kills the Brain (set of videos)
Translating Autism: Informational blog intended to rapidly disseminate the latest scientific findings related to the nature, causes, & treatments of autism spectrum disorders.

Related News

Wired Science, Brandon Keim, October 3, 2007: When is a Conflict of Interest a Conflict?
Arthur Allen, The Huffington Post, September 30, 2007: Congress Finds Nothing Underhanded About CDC's Thimerosal Work
Wired Science, Brandon Keim, September 28, 2007: Thimerosal Question Deserved Better Answer From CDC
David Kirby, The Huffington Post, September 26, 2007: CDC: Mercury in Vaccines Damaged Your Child. Or Not.
Wired Science, Brandon Keim, September 26, 2007: Thimerosal Didn't Fry Children's Brains, New Study Says
Dissident Voice, April 3, 2007: Autism-Mercury Studies Show No Link
A blog by Daedalus, March 26, 2007: Discussion of the false "mercury causes autism" idea
EVISA News, November 16, 2006: A silent pandemic: Industrial chemicals are impairing the brain development of children worldwide
OpEdNews, May 13, 2006: Bush Uses FDA To Shield Big Pharma From Lawsuits
EVISA News, May 3, 2006: Texas Study Relates Autism to Environmental Mercury
EVISA News, March 24, 2006: American lawmakers initiate mercury probe for vaccines
 OpEdNews, July 29, 2005: Autism - Cut The Crap
Robert F. Kennedy, Rolling Stone, June 20, 2005: Deadly Immunity
EVISA News, April 24, 2005: New results about toxicity of thimerosal
EVISA News, February 11, 2005: New findings about Thimerosal Neurotoxicity

Science News, November 13, 2004: Assault on Autism - Scientists target drugs and other environmental agents that may play a role
In These Times, November 11, 2003:  Eli Lilly and Thimerosal

last time modified: June 24, 2020

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