The results of the scientific investigation into an alleged connection between thimerosal and autism have been returned, and they are unanimous and simple. No well designed, peer reviewed study has shown a causal connection between autism and thimerosal exposure (the studies by the Geiers and Mark Blaxill do not count as such). And there have been many studies, by many different research groups, of many different types, including laboratory studies on animals, cohort studies in humans, as well as ecological studies. None has shown even a glimmer of a connection.
Research Studies
One reality that credible scientific researchers understand is that any scientific study can have various problems that undermine its value. The researchers may have a bias that is not understood. The study design may have flaws that are not seen. There may be unexpected confounding factors. Any one study can be erroneous. That is why multiple studies are done on topics of scientific interest. This duplication of research, looking at the scientific topic from many different angles and by many different researchers, promises the best possible results in truth finding. It does not guarantee accuracy. But, it is the best that science (and therefore humans) can do to understand a vastly complex world.
2008 Studies
By 2008, the thimerosal debate had largely been decided. However, one study (Schecter et al.) was completed that looked at autism and thimerosal over a six year period after thimerosal was largely removed from US childhood vaccines. It is discussed in detail in the section applying logic to this debate. It showed that autism rates continued a steady increase even after thimerosal was removed.
2007 Studies
A recent and notable study on thimerosal’s role in neurological function was conducted by the US Centers for Disease Control (“CDC”) (Thompson et al). In fact, just to make sure that those who postulate a thimerosal / autism link felt included, the study authors included in the study design Sallie Bernard (a founding member of SafeMinds, an organization devoted to the belief that mercury poisoning causes autism). She subsequently dissented from the study results after the study did not support the thimerosal / autism link.
The study enrolled 1,047 children between the ages of 7 and 10 and administered tests assessing 42 neuropsychological outcomes, many of which are associated with autism. It assessed the association between current neuropsychological performance and exposure to mercury during the prenatal period, the neonatal period (birth to 28 days) and the first 7 months of life. The study enrolled children from four HMOs that participate in the CDC’s Vaccine Safety Datalink. Birth dates ranged from Jan 1, 1993 to March 30, 1997, during the period of the highest thimerosal exposure in the US. The median cumulative exposure to mercury from thimerosal from birth to 7 months was 112.5 mg (range 0 to 187.5 mg).
Across all three study periods, there was no statistical association between thimerosal exposure and the development of neurodevelopmental abnormalities of the type seen in autism. The only statistically significant associations at all were the following: among boys, there was a significant positive association with performance IQ, and among girls there was a significant negative association with verbal IQ. Higher prenatal exposure to ethyl mercury in boys was associated with significantly better performance on the Stanford-Binet copying test and poorer performance on the WISC-III digit span test of backward recall; among girls, there were no significant associations. Increasing mercury exposure from birth to 7 months among boys was associated with significantly better performance on letter and word identification on the Woodcock-Johnson test, poorer performance on the parental rating of behavioral regulation on the Behavior Rating Inventory of Executive Function, and a higher likelihood of motor and phonic tics. Among girls, higher exposure to mercury from birth to 7 months was associated with significantly better performance on the Grooved Pegboard Test of the non-dominant hand and the WISC-III digit span test of backward recall.
2006 Studies
In a Canadian study on autism and thimerosal, Fombonne et al surveyed 27,749 children born from 1987 to 1998 attending 55 schools in Montreal. Ethyl mercury exposure from vaccines during childhood ranged from 100-125 mcgs from 1987 to 1991 to 200-225 mcgs from 1992 to 1995, to virtually nil from 1996 onwards when thimerosal use was essentially discontinued. The prevalence of pervasive development disorders (autism spectrum disorders) in thimerosal-free birth cohorts was actually significantly higher than that in thimerosal-exposed cohorts (82.7 of 10,000 vs. 59.5 of 10,000). Using logistic regression models of the prevalence data, the researchers found no significant effect of thimerosal exposure in increasing rates of PDD.
2005 Studies
There were no notable thimerosal studies published in 2005.
2004 Studies
A 2004 British study (Heron, Golding et al) studied 12,956 British children who typically had been exposed to ethyl mercury from vaccination at 3, 4 and 6 months of age. It relied on parental responses to questionnaires administered at 7 time points over 91 months, focused on children who were born in 1991 and 1992. Among 69 outcomes, the only adverse association was poorer prosocial behavior, and there were several beneficial associations. Multiple variable analyses done by the researchers found negative associations for thimerosal exposure and conduct behavior, fine motor development, and tics. The authors concluded that the single association (poorer prosocial behavior) they found may be expected given the 69 statistical tests performed.
Another British study completed in 2004 (Andrews, Miller et al) used the UK General Practice Research Database. In this retrospective cohort study, 100,572 term and 2,471 preterm children born from 1988 to 1997 were involved. The thimerosal dose from DPT, the only thimerosal containing vaccine used in the UK, was calculated. Overall, in the term group, 5,831 (2%) neurodevelopmental diagnoses were made, 104 of these being autism and 70 being tics. The only diagnosis for which risk increased significantly with increasing thimerosal dose was tics; autism was not one. For general developmental disorder, unspecified developmental delay, and attention deficit disorder, there was a protective effect associated with thimerosal exposure.
Parker, Schwartz et al conducted a meta-analysis (survey of other studies) in 2004 of studies looking at a link between thimerosal exposure and autism. The authors evaluated and critiqued the methodologies and soundness of the studies conducted to date. The authors concluded “(s)tudies do not demonstrate a link between thimerosal-containing vaccines and ASD (autism spectrum disorders), and the pharmacokinetics of ethyl mercury make such an association less likely”.
A CDC study by Verstraeten (2004) has been much discussed on various blogs. It is a US study that used data from several HMOs to evaluate autism risk and thimerosal exposure. Its author, who reviewed the data several times, concluded that there was no association between autism and thimerosal. There is a conspiracy theory on the internet that the original data showed a significant association between autism and thimerosal and that the CDC covered this data up with the reanalyses. This argument has been debunked.
2003 Studies
In 2003, Stehr-Green et al reported the incidence of autism in Sweden and in Denmark from 1987 to 1999. Both Sweden and Denmark discontinued thimerosal use during the study period, in 1992. The results for both countries were similar. Autism incidence increased throughout the study period and continued to increase (although with some fluctuation) after elimination of thimerosal. The quality of records for autism diagnoses and vaccination rates and the stability of the population studied, characteristic of northern European countries, are strengths of this study.
Also in 2003, Madsen, Lauritsen et al used the same dataset as the Stehr-Green study above but evaluated Denmark only. The study expanded the Denmark information to include 1961-70, when the cumulative ethyl mercury dose was 200 mcgs in the first 15 months of life, and 1970-92, when the dose was 125 mcgs in the first 10 months of life, as well as 1992-2000, when vaccines in Denmark did not contain thimerosal. The incidence of autism was stable until 1990 and thereafter increased throughout the study period.
Another study, this one by Hviid, Stellfild et al (2003), used the Danish Civil Registration System (a thorough database) to examine the rate of autism in children who received thimerosal containing vaccines to those who receive thimerosal free vaccines. In Denmark, the only thimerosal containing vaccine given after 1970 (and prior to 1992) was the DPT vaccine. On the basis of doses given at 5 weeks, 9 weeks, and 10 months of age, a child in Denmark in 1992 and before could have received a total of 125 mcgs of ethyl mercury. The rate for autism for children who received any vaccinations that contained thimerosal as compared to only thimerosal free vaccines was .85 (reduced rate of classic autism for kids who received thimerosal containing vaccines). For other autism spectrum disorders, the rate was 1.12. The authors also evaluated the overall incidence of autism in Denmark after discontinuation of thimerosal in vaccines and found a significant increase per calendar year even after discontinuation of the thimerosal containing vaccines. The organization of the Danish health system lends itself to the type of analysis presented in the study. The cohort includes complete ascertainment of children, developmental diagnoses, and immunizations. Also, children received vaccines from a single manufacturer, the government.
Other Papers (Not Studies) on Thimerosal Not Summarized Here
Mercury, Vaccines, And Autism: One Controversy, Three Histories,
Author: Baker JP
Source: American Journal of Public Health, February 2008;98(2): 244-253
Thimerosal in Vaccines: Balancing the Risk of Adverse Effects with the Risk of Vaccine-Preventable Disease
Authors: Bigham, Copes
Source: Drug Safety, 2005, Vol. 28(2):89-101
Comparison of Blood and Brain Mercury Levels in Infant Monkeys Exposed to Methylmercury or Vaccines Containing Thimerosal
Authors: Burbacher TM, Shen DD, Liberato N, Grant KS, Cernichiari E, Clarkson T
Source: National Institute of Environmental Health Sciences, April 21, 2005
Thimerosal in Vaccines: A Regulatory Perspective WHO Consultation, Geneva, 15-16 April 2002
Authors: Knezevic I, Griffiths E, Reigel F, Dobbelaer R
Source: Vaccine, May 7, 2004, Vol. 22(15-16):1836-41
The Evidence for the Safety of Thimerosal in Newborn and Infant Vaccines
Author: Clements CJ
Source: Vaccine, May 7, 2004, Vol. 22(15-16):1854-1861
The Toxicology of Mercury—Current Exposures and Clinical Manifestations
Authors: Clarkson TW, Magos L, Myers GJ
Source: New England Journal of Medicine, October 30, 2003, Vol. 349(18):1731-7
Impact of the Thimerosal Controversy on Hepatitis B Vaccine Coverage of Infants Born to Women of Unknown Hepatitis B Surface Antigen Status in Michigan
Authors: Biroscak BJ, Fiore AE, Fasano N, Fineis P, Collins MP, Stoltman G
Source: Pediatrics, June 2003, Vol. 111(6):e645-9
Study Fails to Show a Connection Between Thimerosal and Autism
Source: American Academy of Pediatrics, May 16, 2003
Thimerosal and Autism?
Authors: Nelson KB, Bauman ML
Source: Pediatrics, March 2003, Vol. 111(3):674-9
Vaccine Safety Policy Analysis in Three European Countries: The Case of Thimerosal
Authors: Freed GL, Andreae MC, Cowan AE, et al
Source: Health Policy, December 2002, Vol. 62(3):291-307