• Thu, Sep 27 2007

No Causal Association Between Early Exposure to Mercury and Neuropsychological Outcomes

A new study published in the September 27th New England Journal of Medicine does not support a link between early thimerasol exposure and neuropsychological deficits in children. Epidemiologist William V. Thompson, PhD, of the CDC’s National Center for Immunizations and Respiratory Diseases and others, conclude that:

Our study does not support a causal association between early exposure to mercury from thimerasol-containing vaccines and immune globulis and deficits in neuropsycological functioning at the age of 7 to 10 years.

1047 children between the ages of 7 and 10 were administed standardized tests assessing 42 neuropsychological outcomes (autism spectrum disorders were not assessed). The children were from four HMOs that participate in the Centers for Disease Control and Prevention’s Vaccine Safety Datalink. Researchers looked at exposure to mercury during the prenatal period, the neonatal period (birth to 28 days), and the first 7 months of a child’s life; they drew on computerized immunization records, medical records, personal immunization records, and parent interviews to determine exposure to thimerasol; interviews and medical charts provided information about potential confounding factors. Only a few significant associations between early exposure to mercury from thimerasol and neuropsychological outcomes were detected; all were small and “almost equally divided between positive and negative effects.” Specifically:

  • Higher prenatal mercury exposure was associated with better performance on one measure of language; it was associate with poorer performance on one measure of attention and executive functioning.
  • Higher levels of mercury exposure from birth to 7 months was associated with better performance on one measure of fine motor coordination and on one measure of attention and executive functioning.
  • Increasing mercury exposure from birth to 28 days was associated with poorer performance on one measure of speech articulation and better performance on one measure of fine motor coordination.

The researchers noted these limitations regarding their study: only 30% of the subjects included for recruitment were enrolled; they did not control for interventions such as speech therapy; thimerasol exposure beyond 214 days was not assessed. These strengths were noted: the neuropsychological assessment used was similar to that used in the landmark studies of prenatal exposure to methyl mercury; children were enrolled based on their receiving vaccines, rather than on the basis of documented neurodevelopmental diagnoses; exposure information for both mothers and children was collected from a variety of sources. The study concludes:

The overall pattern of results suggests that the significant associations may have been chance findings stemming from the large number of statistical tests that we performed.

While the authors of the study clearly state its parameters and, too, its limitations, news sources and some autism organizations (those that claim that autism is really “mercury poisoning”) are packaging the study as about vaccines and autism, and suggest to what extent these are linked in the public consciousness.

Safe Minds, whose Executive Director, Sallie Bernard, was an external consultant and dissenting member of the study, put out a press release stating that the new study’s “draws a misleading conclusion.” But this statement is as “misleading” as the headlines cited above are about the study’s actual contents. This only furthers the belief that—no matter how clearly it is stated and shown that there is no causal association between early exposure to thimerasol and later neuropsychological outcomes—-a link between these has been firmly established in the public mind. And disputing that link will take more studies, more evidence, more efforts, and more self-scrutiny of why we believe what we believe.

More discussion of the NEJM study and of the responses to it is at Respectful Insolence and Left Brain/Right Brain, and by Arthur Allen.

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  • Junior

    “And disputing that link will take more studies, more evidence, more efforts, and more self-scrutiny of why we believe what we believe.”

    I’m beginning to wonder if there is anything that will convince some people. This question is to those who believe in an autism/vaccine connection. What would it take to convince you that there isn’t a connection? Do you think there is anything that could convince you there isn’t a connection?

  • María Luján

    The amount of new research published on clinical aspects in autism is very important, each month.
    The quantification and analysis of adverse effects of vaccines should be done considering clinical,immunological, nutricional gastroenterological and genetic information.
    The problem with the manuscripts that reassure safety of vaccines are
    1-they are based on healthy people trials
    2-there is no multidisciplinary clinical analysis of the health status
    Therefore the evidence is partial, non-conclusive and needed of further analysis, especially in subgroups of susceptible people.
    Being the studies done on a general population, why the detection of 1 in 150 with different genetics is going to be possible if genetic epidemiology is NOT being done?
    There are no consensuaded study/method to know clinically if a child/teen/adult has bioaccumulation of heavy metals (Hg/Pb/others) and other toxic elements (Al); there are no studies on the genetic and biochemistry related to the transport /excretion of toxic elements in autism
    What is the value of studies done mixing completely the genetic, nutritional, immunological and gastroenterological status of the participants?
    Why someone is going to be convinced of something that has not been studied yet?
    Why someone is going to be conviced of anything-negative of affirmative with this situatios?
    It is a circular situation

  • Chuck

    There is no scientific study that can determine causation of a disorder that has a subjectively determined list of diagnosis criteria and can also be subjectively diagnosed.

  • C

    What we need is a randomized trial in which one group of newborns recieves placebos instead of vaccines, and the other group of infants recieves vaccines as reccomended, double blinded of course. Recruiting 1,000 infants per group, multi sites, follow the tykes for 7 years, and no Internal Review Board would ever approve it.

    However, as provaccine as I am, I would be interested to know if any studies have been done regarding genetically susceptible infants (younger siblings of autistic children for example) and vaccines. Anyone know of a such a study?

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  • isles

    C, I believe such a study is ongoing in California. Of course it is observational rather than interventional; it would be unethical to knowingly cause a child to be vulnerable to vaccine-preventable diseases.

  • María Luján

    This manuscript presents several ideas on the kind of studies and problems related
    Causal models in epidemiology
    [quote]Ignorance about steps in a causal chain will hamper the identification of component causes, whether environmental or genetic, in that chain[/quote]

  • passionlessDrone

    Hi C –

    “However, as provaccine as I am, I would be interested to know if any studies have been done regarding genetically susceptible infants (younger siblings of autistic children for example) and vaccines. Anyone know of a such a study?”

    This study is being performed every day by parents of autistic children who decide not to vaccinate their next offspring. It does not meet the double blinding criteria, however. There are plenty of parents of autistic children who continue vaccinating though, so numerically there should be enough to sample from.

    The problem, it would seem, would be in getting people to turn out for the analysis phase without bias.

    - pD

  • C

    No problem I LOVE the analysis phase (really).

    And pD you have hit the nail on the head: antecdotal information from parents and concerned others is biased and not collected in a systematic manner. Seriously, I would be very interested in a study of higher risk children (for autism) and their vaccine patterns.

    And thanks for the CA study info, yep the unethical part certainly is a stopper (go IRBs).

  • http://www.autismvox.com Kristina Chew, PhD

    No matter how many studies are done, it does seem that the dispute may be neverending, as noted in a story in Newsweek this week.

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  • http://www.mumkeepingsane.blogspot.com Leanne

    I’ve often wondered if it might be helpful to study older siblings of autistic children. My oldest son is NT and I’ve long wondered why that is so (more often actually than wondering why my youngest is autistic).

    I agree strongly with what Chuck said as well.

  • Chuck

    What would the results of an individual that has been thoroughly reviewed by two different neurologists as having no noteworthy neurological deficits and having ASD be for this study?

  • María Luján

    There are several studies on siblings of autistic children:
    J Autism Dev Disord. 2007 Jan;37(1):145-57. Epub 2007 Jan 11.
    Early social, imitation, play, and language abilities of young non-autistic siblings of children with autism.Toth K, Dawson G, Meltzoff AN, Greenson J, Fein D.
    UW Autism Center and Center on Human Development and Disability, University of Washington, Seattle, USA.

    Studies are needed to better understand the broad autism phenotype in young siblings of children with autism. Cognitive, adaptive, social, imitation, play, and language abilities were examined in 42 non-autistic siblings and 20 toddlers with no family history of autism, ages 18-27 months. Siblings, as a group, were below average in expressive language and composite IQ, had lower mean receptive language, adaptive behavior, and social communication skills, and used fewer words, distal gestures, and responsive social smiles than comparison children. Additionally, parents reported social impairments in siblings by 13 months of age. These results suggest that the development of young non-autistic siblings is affected at an early age and, thus, should be closely monitored, with appropriate interventions implemented as needed.
    I have also wondered how a vaccinated sibling can affect a non-vaccinated brother/sister -autistic-and how the contact with also wild infections can affect- in this case- the non-vaccinated child ( if susceptible). AS others said, a study- in immunological terms-testing IgG/IgM for different vaccine preventable diseases – and herpes group virus for example-could be a way to study this group-in terms of contact and impact with/of infections.

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