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.