A FRESH LOOK

Congress has asked the National Institute of Environmental Health Sciences (“NIEHS”) to take a fresh look at the possible link between thimerosal and the development of autism. A letter sent to the NIEHS by eight members of Congress (Senators Joseph Liberman and Debbie Stabenow; and Representatives Dave Weldon, Dan Burton, Carolyn Maloney, Joseph Crowley, Chris Smith, and Maurice Hinchey) explains the reasoning. It reads in part:

We hope that NIEHS can bring its expertise to the issue of thimerosal. Many of us in Congress are concerned about the possibility of an association between autism and thimerosal in vaccines. While we understand that the Institute of Medicine (IOM) has determined that the evidence thus far does not support an association, we know that for too many the issue is still not resolved. The exposure of American children from 1990to 2001 to thimerosal was unique it was 75 percent greater than their European counterparts who are the focus of the majority of the epidemiological research on this issue upon which the IOM relied. This period coincided with an autism epidemic now affecting I out of every 166 children. Additionally, thimerosal is by weight 50 percent ethylmercury, which is a potent neurotoxin. The level of thimerosal in these vaccines exceeded the EPA oral daily dose limit for methytmercury by over 120 times at the 2, 4, and 6 month pediatric visit. Therefore, we believe that there is room for a closer look on any potential association between thirnerosal exposure and the risk of autism, both to learn new information and to solidify the public’s trust in vaccines.

The Congressional letter notes the IOM’s reliance on the Vaccine Safety Datalink (“VSD”), maintained by the Centers for Disease Control (“CDC”). Indeed, the big complaint with a recently published study by the Geiers is that they used a less-reliable source of data. The clear intent of Congress is that we compare apples to apples in determining whether the reduction of thimerosal use in vaccinations has had an impact:

Thus far, with regard to thimerosal and autism, the CDC has studied and reported on this database only once. That study was based on data collected prior to the removal of thimerosal and failed to explicitly compare the outcomes of children who received thimerosal-containing vaccines (TCVs) with those who did not. The VSD now includes live years of additional data. We are aware that the CDC is presently preparing to conduct further analysis of the VSD to investigate the post-thimerosal data. Unfortunately, as the lOM pointed out in its April 2005 report on VSD access, a CDC-led study on thimerosal and autism could be viewed with much skepticism and may not he accepted by the growing number of parents with concerns about vaccine safety and the possible links between thimerosal and autism. If the federal government is going to have a study whose results will he broadly accepted, such a study cannot be led by the CDC. It is for these reasons we believe that NIEHS is the most appropriate entity to lead such an investigation.

Specifically, Congress has asked the NIEHS to convene a workshop with representatives of “the CDC, the autism community, members of the public concerned about vaccine safety research, and external academic researchers” so that everyone can try to agree on how to use the VSD information to answer the big question: has thimerosal contributed to the increased incidence of autism?

This will be an important inquiry, to be sure. It’s hard, though, to get too excited about the prospects. First, the Congressional letter was not completely accurate when it referred to the removal of thimerosal from vaccines. As is well-known, thimerosal is still widely used as a preservative in most flu vaccines. Moreover, the commonly held belief that most “preservative-free” vaccines are “thimerosal-free” has been challenged. Although thimerosal is not used as a preservative is most vaccines, it is still used as part of the manufacturing process of some vaccines. Filtering the thimerosal out may be an imperfect exercise, and there is an ongoing debate as to the exact quantity of the “trace amounts” that are being found.

Still, it cannot be denied that the thimerosal exposure in today’s vaccines is negligible compared to what it was. And so the numbers should be going down if thimerosal is a causative factor. That does not necessarily mean there are five years of post-thimerosal data available. Considering the shelf life of the vaccines manufactured prior to the phase out, along with the delay in diagnosis for autism, there may only be a year or two of solid numbers, which may not be enough to show a real trend.

Another thing that needs to be addressed is the combined impact thimerosal may have with other factors, including live-virus vaccines. If the input from the “autism community” is limited to concerns over whether “autism is a misdiagnosis for mercury poisoning,” the study may well miss the possibility that thimerosal exposure is but one in a series of insults to the immune system that may eventually lead to a diagnosis of autism.

I remain skeptical that even a well-designed epidemiological study can prove or disprove biological causation. Nevertheless, to the extent that epidemiology can provide important clues, I am happy that we may have a truly independent study.