There has been much discussion – and some confusion – about a letter sent earlier this year from CDC Director Dr. Julie Gerberding to Congress, in response to a report by a panel of the National Institutes of Environmental Health Sciences (NIEHS) that detected many weaknesses and limitations with the CDC-run Vaccine Safety Datalink (VSD).
In her letter, Dr. Gerberding wrote that “CDC concurs” that the weaknesses and limitations inherent in the database would call into question the usefulness of using the VSD for conducting ecological and other types of studies on the potential association between thimerosal exposure and autism risks.
It has been my contention that the same weaknesses and limitations that reduce the usefulness of VSD-based ecological studies going into the future, would likewise have negatively impacted the study already conducted by Thomas Verstraeten, which was a retrospective cohort study, and not an ecological one.
During my briefing in Washington on September 24, I mentioned the NIEHS report, and included the CDC agreement with the findings, without explaining the important distinction between an ecological study and a retrospective cohort study.
During the Q&A session, a Congressional staff person asked me about this, and suggested that I had tried to conceal information and thus deceive people in the room. This was not my intention.
On Tuesday, September 30, I sent that staff person the following letter, which reprises and expands upon the answer I gave in person on the 24th.
I have decided to reprint it here, because I think it is important to note that Dr. Gerberding did indeed concur with findings of “limitations and weaknesses” within the VSD database which would negatively impact ecological studies going into the future.
These problems woud ALSO impact other VSD-based studies (i.e. Verstraeten) conducted in the past. Sadly, but perhaps not surprisingly, Dr. Gerberding in her letter to the powerful House Appropriations Committee, utterly failed in her duty as CDC Director to address, or even respond to, the most serious and important finding in the NIEHS report. Namely, that the fundamental “weaknesses” within the VSD itself “reduce the usefulness” of the VSD to study thimerosal and autism. Period.
This is important, of course, because the Verstraeten study was a cornerstone of the 2004 Institute of Medicine report, which found no evidence of a link between thimerosal and autism. It remains a pillar of medical opinion to this day, despite a letter from Verstraeten himself, published in Pediatrics, insisting that his study found no evidence AGAINST an association, and was in fact a neutral study, and that indeed, more research was recommended.
Here is the letter. I will let readers know when I get a reply:
SEPTEMBER 30, 2008
Thank you for attending the Congressional briefing on Autism on September 24. I was glad to see that you took the time out of busy week to listen. I also appreciate your question about the NIEHS report on the VSD database, HERE and Dr. Julie Gerberding’s response to the House Appropriations Committee.
As you rightly pointed out (and as I concurred that day) I omitted an important detail in regards to Dr. Gerberdings’s letter to the Committee. I regret that, and never meant to mislead people in the room.
It was a rather artless sin of omission.
I think the lesson for me here is that, when you try to cram a two hour presentation into 25 minutes, it is wise to not include very complicated and, as you put it, “somewhat arcane” details that are difficult to explain in such a short period of time. In retrospect, I probably should have focused solely on the NIEHS report itself, and left the Gerberding letter out of the presentation entirely.
That said, I still assert that the NIEHS report was very much germane the Verstraeten study, because it indentified, (as I said in my response to you on Wednesday), serious structural weaknesses inherent in the database, which might impact any type of VSD-based study to determine the risk of autism, if any, associated with thimerosal exposure.
The NIEHS panel’s mission, essentially, was twofold. The first was to evaluate the database itself for strengths and weaknesses for doing thimerosal/autism studies of any kind. The second, (broken down into specific ideas) was to evaluate the feasibility of using the database for certain studies, including ecological ones, going into the future.
As I interpret things, the panel concluded that the database itself suffered from several weaknesses and limitations, which in turn reduced its usefulness for studies of autism risks from thimerosal (ie, Verstraeten) AND ALSO reduced the feasibility of future studies (ie, ecological ones) that are based on data collected within the VSD.
In the NIEHS report, there was a charge to the panel that listed five distinct tasks:
1) Identify the strengths and weaknesses of the VSD for evaluating the possible association between exposures to thimerosal-containing vaccines and AD/ASD
2) Advise NIEHS and CDC on the feasibility of a new VSD study to compare autism rates before and after removal of thimerosal from most US childhood vaccines, using an ecologic study (an epidemiologic design where there is no linkage between individual-level data on exposure and health outcome)
3) Identify any other uses of the VSD or other existing resources that might be used to examine an association between thimerosal and AD/ASD.
4) Develop recommendations for design, conduct, analysis and oversight of any proposed study.
5) Discuss the potential impact of possible VSD-based studies in the context of what is already known about autism and ASD.
In the “Report of the Panel,” the experts convened by NIEHS addressed Task #1 at the very beginning, and then went on to provide details of limitations and weaknesses, in the context of potential future (ie, ecological) studies.
The panel wrote: “Despite these overall strengths, the panel identified several areas of weakness. The cumulative effect of these weaknesses was judged to reduce the usefulness of the VSD for addressing the potential association between exposure to the vaccine preservative thimerosal and risk of AD/ASD. The weaknesses of primary importance are summarized below.”
Again, I interpret that to mean that the VSD itself suffers from limitations and weaknesses, regardless of what type of thimerosal-autism study it might inform.
Unfortunately, in her letter to the Committee, Dr. Gerberding chose to ignore this very serious first finding, and instead addressed the other concerns about using the VSD for ecological and other types of investigations.
Even so, Dr. Gerberding did concur that many of the weaknesses cited by the panel (for example, under-ascertainment of cases, changing business practices, differing diagnostic codes, and no accounting for prenatal and background mercury exposure) would reduce the usefulness of the VSD in conducting ecological studies of thimerosal and autism risks.
My point (and terribly made, I admit) was that Dr. Gerberding agreed that the VSD database contains many limitations and weaknesses, making it less than useful for doing ecological studies. However, these same problems would have, and indeed did, affect the Verstaeten study.
Finally, Dr. Gerberding wrote that CDC does not conduct ecological studies using the VSD. Instead, she (falsely) stated that CDC uses chart reviews, neurological follow up exams, and parental interviews in all VSD-based thimerosal studies.
The problem is, Verstraeten had no parental interviews and no neurological follow up exams. And there was very limited chart review: The investigators reviewed the carts of roughly 1% of the kids in the study.
So even though Verstraeten was not an ecological study, it did suffer from some of the same weaknesses that would impact an ecological study. The weaknesses that the NIEHS found within the database, and to which the CDC admits, call into question the validity of the Verstraeten study – again, in my opinion.
This is, indeed, arcane. And it is why I urged everyone to watch the DVD of my NYU talk, for a more thorough explanation. I have no reason to deceive people. I know that credibility means everything in this discussion, and I try to protect my own as much as possible.
I enclose a link to my remarks on this subject at NYU (which has also been added to the transcript on Age of Autism) HERE.
And HERE is a link to my Huffington Post piece on this issue, which as you can see, I corrected (the next day) in bold at the top of the piece.
Finally, I would very much like to speak with you on the phone, and go over the NIEHS report, to see if you interpret it differently than I do. Perhaps you can convince me that the inherent weaknesses in the database (to which Dr. Gerberding admits) are in no way applicable to the Verstraeten study. I promise to keep an open mind!
Many thanks, again, for attending the briefing, and for your thoughtful and detailed approach to this discussion.
Very respectfully yours,
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