Nov 27 2009

Answering Some Autism Questions

Like my science-blogging colleagues, I get e-mail. I always appreciate it when readers (or listeners, in the case of the SGU) take the time to write. Sometimes the e-mails are questions from someone who disagrees with my position on a hot issue. I especially like these e-mails – they are good blog fodder, and I think the format of answering questions is more compelling and interesting than making a didactic argument.

Below is an example of the kind of question I most like to get – from someone who disagrees with me, but still manages to ask polite and cogent questions. This stands in stark contract to most hostile e-mail I get, which are just strings of ad hominems, straw men, and other logical fallacies. I get the impression (and some of my e-mailers have later even admitted this) that the e-mails were not meant as an opening to serious discussion, but as a venting rant into the ether of the internet.

Harold asks some very important questions about the alleged autism-vaccine link and research priorities, and I am happy for the opportunity to clarify my position. His e-mail begins below the fold:

Dear Dr. Novella

I am a parent of a 13 year old boy with Autistic Disorder. I also have a blog site on which I commented unfavorably on your response to the possibility that the IACC might recommend some vaccine autism research. I ask your response to a few questions if you have the time and are inclined to respond.

One is your apparent opposition to any further research exploring possible vaccine autism connections. I am not an “anti-vaxxer”. I have never attributed my son’s autism to vaccines. Until recently I accepted the official view that vaccines play no role in causing autism. More recently my views have moved toward an undecided position. This change began when reading Dr. Bernadine Healy’s observations about the limitations of the epidemiological studies which are usually used to allegedly “debunk” any vaccine autism connection. In her comments she indicated that such studies are not specific enough to address the possible impact of vaccines or their ingredients on potentially vulnerable population subsets.To this layperson Dr. Healy’s criticism seems reasonable as does her call, a call also made by Dr. Julie Gerberding, that an observational study comparing autism rates in existing vaccinated and non vaccinated populations could and should be done. Your opposition to further vaccine autism study does not, with respect, seem either reasonable OR science based. I ask if you could provide a clear rationale for opposing a study which has been called for by two prominent health authorities and which might provide useful information to what is a heated debate on all sides.

The second question I have for you concerns the increases in autism diagnoses which has really been quite startling by any measure. In my son’s lifetime the figure has changed from 1 in 500 to 1 in 150 with the two recent studies indicating it might now be 1 in 91. Many authorities dispute though that the increases are real pointing to the autism definition changes in the DSM and ICD diagnostic manuals in the 1993-4 period and increased social awareness as the reasons for the increases in diagnoses. Is it your view that the increase in autism diagnoses does not reflect a real increase in autism disorders? If so what are the implications of that position for the argument that epidemiological studies have disproved any thimerosal vaccine link because autism rates increased after removal of thimerosal from MOST vaccines?

Also if you have the time and inclination do you think Dr. Healy’s observations that thimerosal which continued to be found in flu vaccines, some of which historically were administered to pregnant mothers, was a matter worthy of investigation given that the thimerosal crosses the placenta is a legitimate concern worthy of further investigation? This is an important matter here in Canada where the squalene adjuvant was removed from the vaccines given to pregnant women but there has been no indication that thimerosal has been removed.

I would genuinely appreciate your responses to these questions which I am posting on my blog site. If you do me the courtesy of an informed reply I would be happy to post that reply as well.


Harold L Doherty

And here is my response:


Thanks for your thoughtful e-mail. I would be happy to address your questions.

Regarding further research – it is always possible, in the face of negative results, to call for still more research. And it is easy to make this seem like the default scientific position. ESP proponents, after a century of failed research, call for still more research, and accuse anyone who says further research is not worthwhile of being unscientific.

Also, to clarify my position, I am not categorically against further research into vaccines and autism. I think any such research is most likely to confirm the lack of a correlation. And if there is a susceptible sub-population, of course it would be good to know about it to make the vaccine program even safer. (But keep in mind – it is a common form of special pleading to argue that, when the effect you are looking for is absent, that it only exists is a subpopulation that existing data was not powerful enough to detect. This may be true, but it is just post-hoc speculation, and doesn’t change the fact that the data is negative.)

My position is that ideological groups should not be dictating how scarce research funds are allocated. When we put money, people, and resources into chasing down an unlikely hypothesis those resources are not available for what might be more promising research. My position is that objective scientists, justifying their position with a careful analysis of the research, should decide how best to allocate scarce research funds. The anti-vax movement, however, is trying desperately to put their thumb on the scale – and that is what I oppose. They are trying to subvert autism activism to serve an anti-vaccine agenda – and they are hurting the autism community as a result, in my opinion.

I respect Healy and Gerberding, but I disagree with their approach in that they think more research will satisfy vaccine critics, but this is a naive position. The anti-vaccine movement has already demonstrated that they are impervious to facts and evidence, and spending time and money trying to placate them is a fool’s errand. The CDC even went as far as to include them in designing a trial looking at vaccines and neurological disorders, and then only after the results came back negative, did they criticize the study.

You specifically mention an observational study comparing vaccinated and unvaccinated children – I do not oppose this. If it can be done with reasonable resources, there are scientists willing to carry out such a study and think it is worthwhile, and the results will be useful, then I fully support it and await the results. (And in fact I have never opposed such studies.) What I and others have written is that an experimental (not observation) comparison of vaccinated vs unvaccinated children is unethical, because it would randomize children to not get standard preventive care, and that directly violates human research ethical guidelines. Observational studies are fine, but they are never definitive, and they will not, in my opinion, change or end the debate. They will not move the anti-vaccinationists one bit.

The increase in autism diagnoses has been studied from multiple angles – not just the expansion of the diagnosis.

You will find a summary of relevant research here:

So far, every way it has been looked at the hypothesis that the increase in diagnosis of ASD is due to increased surveillance and expanded diagnosis has been confirmed. There is evidence of diagnostic substitution (as ASD numbers increase, the numbers of other similar diagnoses decrease). There is evidence that different age groups have the same prevalence of ASD (rather than increasing with younger age, as would result from a true increase in ASD). And if you apply the same diagnostic and surveillance methods to a cohort over time, you get the same ASD prevalence. The data is actually quite convincing that true autism rates are not significantly increasing (you cannot rule out a small real increase, or decrease for that matter, that the data is not powerful enough to detect) but that there has been expanded diagnosis with diagnostic substitution and increased surveillance.

I discuss this further here ( and here (

The implications of this position to the alleged thimerosal link are complex, but support a lack of correlation. What the data shows is that in various countries ASD diagnoses began to rise around the same time (as diagnostic patterns changed), in the early 1990s, and have continued to rise through today. Meanwhile, vaccine policies have varied considerably with regard to total thimerosal dose, with several countries, at different times, removing most thimerosal from vaccines. Every study looking at the data shows no correlation between the steadily increasing ASD diagnostic rates with the rising and falling thimerosal doses at different times in different countries. This is powerful evidence for a lack of correlation. As you likely know, toxicity is always about dose, and seeing a proper dose-response is essential to proving toxicity causation. What we have with thimerosal is an absolute lack of any dose-response, in many studies and sets of data.

Also, please keep in mind that the anti-vaccine movement used the increase in ASD in the 1990s as their original justification for the claim that thimerosal causes autism. They predicted that autism rates would decline after thimerosal was removed from the childhood vaccine schedule, and we agreed that if that happened we would need to rethink the possibility of a connection. Well, rates continued to rise without a blip, effectively putting the final nail in the coffin of the thimerosal hypothesis.

Regarding thimerosal and pregnancy, to the extent that this has already been studied (again, you can find references in the vaccine-and-autism link above) there has been no correlation. I do not oppose further research, however, if the CDC or others think it is warranted and feasible.

Thanks again for the interesting questions, and I hope this adequately clarifies my position.


Steve Novella

18 responses so far