Fourteen Studies

Wins award for most conflicts with seven different vaccine companies mentioned. But, study doesn't even look at autism as an outcome, so doesn't apply to this debate at all. Had a panel member dissent from study conclusions, a bad sign. May be interesting to some, but not anyone studying autism.

Q: Did exposure to thimerosal in early childhood impact certain neurological outcomes, but not autism?

"Dr. Thompson reports being a former employee of Merck; Dr. Marcy, receiving consulting fees from Merck, Sanofi Pasteur, GlaxoSmithKline, and MedImmune; Dr. Jackson, receiving grant support from Wyeth, Sanofi Pasteur, GlaxoSmithKline, and Novartis, lecture fees from Sanofi Pasteur, and consulting fees from Wyeth and Abbott and serving as a consultant to the FDA Vaccines and Related Biological Products Advisory Committee; Dr. Lieu, serving as a consultant to the CDC Advisory Committee on Immunization Practices; Dr. Black, receiving consulting fees from MedImmune, GlaxoSmithKline, Novartis, and Merck and grant support from MedImmune, GlaxoSmithKline, Aventis, Merck, and Novartis; and Dr. Davis receiving consulting fees from Merck and grant support from Merck and GlaxoSmith- Kline. No other potential conflict of interest relevant to this article was reported."

Scoring (Out of 40 possible points):

Asked the Right Question: 0

Ability to Generalize: 4

Conflict of Interest: 0

Post-Publication Criticism: 1

Total Score: 5

"Of 3648 children selected for recruitment, 1107 (30.3%) were tested. Among children who were not tested, 512 did not meet one or more of the eligibility criteria, 1026 could not be located, and 44 had scheduling difficulties; in addition, the mothers of 959 children declined to participate."

Participation level was low, could have excluded children with adverse outcomes.

To the Editor: Thompson et al. (Sept. 27 issue) 1 report the results of a study investigating the neuropsychological outcomes of early exposure to thimerosal. As a dissenting member of the panel of external consultants for this study, I object to the authors' conclusion that there is no causal association between thimerosal and children's brain function. The sample comprised children who were least likely to exhibit neuropsychological impairments. Specifically, children with congenital problems, those from multiple births, those of low birth weight, and those not living with their biological mother were excluded. The sample was skewed toward higher socioeconomic status and maternal education -- factors that are associated with lower rates of neurobehavioral problems and higher intervention rates and that were not measured. The sampling frame included only children enrolled from birth in the health maintenance organization (HMO) and still enrolled after 7 to 10 years, excluding children in higher-mobility families, who tend to have lower academic and behavioral function. 2 Children with neurobehavioral problems may have been less likely to remain with the HMO. Only 30% of families selected for recruitment participated, a low rate for scientific research. Among the families selected for recruitment, 26% refused to participate. Another 28% "could not be located," which included families that did not respond to multiple recruitment attempts (internal documentation from the study contractor, Abt Associates) -- another form of refusal.

1. Thompson WW, Price C, Goodson B, et al. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med 2007;357:1281-1292. [Free Full Text]

Autism Speaks Statement Regarding "Early Thimerosal Exposure and Neuropsychological Outcomes at 7-10 years" in the New England Journal of Medicine
September 26, 2007

This afternoon the New England Journal of Medicine published a CDC study examining the hypothesis that exposure to thimerosal during early development is associated with neuropsychological deficits in children. The authors of the report, entitled "Early Thimerosal Exposure and Neuropyschological Outcomes at 7 to 10 Years," concluded that findings from the study do not "support a causal association between early exposure to mercury from thimerosal-containing vaccines and immune globulins and deficits in neuropsychological functioning at age of 7 to 10 years."

While the study conclusions are supported by the data presented, the study had significant limitations that argue for the need for additional study. The researchers acknowledged there are limitations to the study, including a 30% recruitment rate, which did not allow them to adequately control for potential selection bias, and difficulty in controlling for treatment or intervention, such as speech therapy, which could potentially lessen or compensate for the possible negative effects of thimerosal exposure.

Among the 42 neuropsychological parameters examined, only a small number of very modest associations were detected. For instance, higher thimerosal exposure was found to be associated with poorer performance in some areas, such as behavioral regulation and tics in boys, and better performance in other measures, such as performance IQ and fine motor coordination. The authors highlighted these findings in their discussion because similar associations had been reported in a few earlier studies and suggested "the potential need for further studies." Since the reported associations are weak and essentially equally divided between positive and negative effects, the investigators concluded that they are more likely due to chance or statistical artifacts and do not lend meaningful support to a causal connection between thimerosal and neuropsychological deficits in children.

Although the general study conclusions are supported by the data presented and are consistent with past findings, given the significant study limitations and some of the intriguing albeit inconclusive associations involving behavioral regulation and tics, this study isn't and shouldn't be seen as the "last word" on the topic. If anything, it is a great example why we must take a systematic, rigorous approach to the science involved if there is ever going to be hope for a compelling and satisfactory answer. Just as important is the understanding that in science, it is rare that any given study would deliver a definitive conclusion. 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.

While the study does not specifically examine the link between thimerosal and autism spectrum disorders, it does explore neuropsychological functioning, such as language development, attention, and fine motor coordination, that are affected in some individuals with autism. The Centers for Disease Control are currently studying the potential association between autism and thimerosal and are expected to report findings next year.