Jan 27 2009

Yet More Evidence Against a Link Between Thimerosal and Autism

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Comments: 28

A new study published yesterday (Monday) in the journal Pediatrics provides more evidence against any link between thimerosal (a mercury-based preservative in some vaccines) and autism or other neurological disorders. This study adds to the large and growing body of scientific evidence for the safety of vaccines, and contradicting the claims of the anti-vaccine movement that vaccines cause autism.

The study is a bit fortuitous in that it was not originally designed to probe this question. Rather, this was a safety and efficacy study of the acellular pertussis vaccine conducted in Italy between 1992 and 1993. But it created a cohort of children who were carefully screened and monitored, and randomized to different exposures to thimerosal. This allowed the researchers to go back 10 years later to survey and examine the children for neurological disorders.

Here are the methods:

Children who were enrolled in an efficacy trial of pertussis vaccines in 1992–1993 were contacted in 2003. Two groups of children were identified, according to thimerosal content in vaccines assigned randomly in the first year of life (cumulative ethylmercury intake of 62.5 or 137.5 µg), and were compared with respect to neuropsychological outcomes. Eleven standardized neuropsychological tests, for a total of 24 outcomes, were administered to children during school hours. Mean scores of neuropsychological tests in the domains of memory and learning, attention, executive functions, visuospatial functions, language, and motor skills were compared according to thimerosal exposure and gender. Standard regression coefficients obtained through multivariate linear regression analyses were used as a measure of effect.

The results – there was only one case of autism in the entire study, and that was in the group that received the lower dose of thimerosal. Of the 24 neuropsychological tests, only two were significantly worse in the higher dose thimerosal group:

Girls with higher thimerosal intake had lower mean scores in the finger-tapping test with the dominant hand and in the Boston Naming Test.

The differences were clinically small, even though they were statistically significant. However, by chance alone (with the statistics used) 4 outcomes on average should have varied significantly, therefore the fact that 2 outcomes were significant is almost certainly due to chance alone. Further, these same measures were not different for boys or the groups as a whole, only when girls are considered seperately, which further suggests it is statistical chance.

And finally, a previous study by the CDC that also looked at a large number of neurological measures also found a few statistical outliers – but they were different outcomes. For example, the previous CDC study showed that motor tics were increased in the thimerosal exposure group, while this study shows no difference in motor tics but does show a decrease in finger tapping. This is all consistent with statistical noise – not a real effect.

The study authors conclude:

No study conducted to date has been able to provide conclusive evidence of an effect of thimerosal on neuropsychological development. Final judgments regarding this association must rely on the entire body of results from studies conducted in different settings and with different levels of validity and on the coherence of results. The lack of consistency among the results of our study and other available studies suggests that an association between thimerosal exposure through vaccination in infancy and neuropsychological deficits is unlikely or clinically negligible. Additional data from populations with wider ranges of exposure to thimerosal and additional neuropsychological assessments at older ages may help to clarify the issue of potential associations between thimerosal and neurodevelopmental outcomes.

This is appropriately conservative prose for a technical paper, but even so the conclusion is solid – any association between thimerosal and neurological disorders is “unlikely or clinically negligible.”

Of course, no one study by itself is definitive, especially in establishing the absence of a correlation. But this new study adds to an already large body of evidence, and therefore raises our confidence further that thimerosal in vaccines did not cause autism or other neurological disorders.

The authors are also candid about the strengths and weaknesses of the study. Anti-vaccinationists have been calling for years for a randomized study with and without vaccines – because that is the one type of data that was not available. It is unethical to randomize a patient not to receive a standard therapy.

This study, however, is close because subjects were randomized to two very different doses of thimerosal.

Another likely source of criticism is that the total cumulative dose of ethylmercury in the higher exposure group was 137.5 µg and that this might be below the threshold of toxicity, and therefore says nothing about the risks of higher exposures. This is true, but not likely to be relevant to the thimerosal question. In the US thimerosal exposure was about 75 µg in 1989 and peaked in 1997 at 187.5 µg cumulative exposure through the routine childhood schedule.

It does not seem plausible that the increase in µg of ethylmercury from 75 to 187.5 resulted in a dramatic increase in autism, but the difference between 62.5 and 137.5 in this study was sub-threshold. Also, I have to point out that since 2000 the use of thimersal in the US has decreased and now there are less than 3 µg in the childhood schedule and the rate of increase in autism diagnoses has not decreased even six years later.

Conclusion

This study adds to the consensus of existing evidence for a lack of association between thimerosal in vaccines and autism or other neurological disorders. However, since the anti-vaccination movement is largely ideological and disconnected from the scientific evidence I do not anticipate that this will end the controversy.

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28 responses so far

28 Responses to “Yet More Evidence Against a Link Between Thimerosal and Autism”

  1. PaulGon 27 Jan 2009 at 8:25 am

    Haven’t had time to read the whole thing, but one part of the abstract leapt out immediately – out of a sample population of 1403 with 24 “neuropsychological outcomes”, only two could be significantly associated with thimerosal exposure.

    Sounds about as conclusive as you could ever hope to get by just looking at the numbers for a correlation.

    Sadly, you’re probably right Dr. N. It’ll only convince the already convinced.

  2. [...] fact is, none of this seems to matter. Steven Novella has a post today about a new study that adds to the research showing that the preservative thimerosal is not [...]

  3. thandon 27 Jan 2009 at 1:38 pm

    Is the couching of the results- essentially no link- something Italian studies usually contain, or are there other reasons for the chosen language? (Artifact of translation, perhaps?)

    A few months ago, I wrote a quick story about how I saw these anti-vax “debates”. It currently sits at:
    http://tomhand.blogspot.com/2008/12/story-time.html

  4. HHCon 27 Jan 2009 at 1:41 pm

    Italian research and fashion design- belle arti!

  5. Watcheron 27 Jan 2009 at 4:38 pm

    Anti-vaccinationism has turned into one of those things where if you say it with enough conviction and belief, people automatically believe it … it’s insanity.

  6. Perky Skepticon 27 Jan 2009 at 5:02 pm

    I came sooooooooo close to believing in it, myself, Watcher. Scary, scary! Thank goodness for the skeptical blogosphere, or I might have drunk that Kool-aid when my son was born. I was a Sears follower, and he paints a very woo-friendly picture of childcare. That point of view left me vulnerable to a lot of the “toxin” scares perpetrated by circulating emails. I am just SO GLAD the science is in, for the people who want to see it!

  7. JonoBon 27 Jan 2009 at 6:14 pm

    “It does not seem plausible that the increase in µg of methylmercury from 75 to 187.5 resulted in a dramatic increase in autism, but the difference between 62.5 and 137.5 in this study was sub-threshold.”

    Correct me if I am wrong but by ‘methylmercury’ did you not intend ‘ethylmercury’?

  8. HHCon 27 Jan 2009 at 9:05 pm

    thand, The article is in English, on-line in Pediatrics, Volume 123, Number 2, February 2009, pages 475-482. Dr. Novella’s title for this blog is not the title of the article.

  9. HHCon 27 Jan 2009 at 9:11 pm

    The title of the Pediatrics article is Neuropsychological Performance 10 Years After Immunization in Infancy with Thimerosal-Containing Vaccines.

  10. [...] NeuroLogica Blog » Yet More Evidence Against a Link Between Thimerosal and Autism The study is a bit fortuitous in that it was not originally designed to probe this question. Rather, this was a safety and efficacy study of the acellular pertussis vaccine conducted in Italy between 1992 and 1993. But it created a cohort of children who were carefully screened and monitored, and randomized to different exposures to thimerosal. This allowed the researchers to go back 10 years later to survey and examine the children for neurological disorders. (tags: medicine autism study vaccination) This entry was written by The Prophet King, posted on January 27, 2009 at 10:02 pm, filed under links. Bookmark the permalink. Follow any comments here with the RSS feed for this post. Post a comment or leave a trackback: Trackback URL. « links for 2009-01-24 [...]

  11. Steven Novellaon 28 Jan 2009 at 9:40 am

    Correct me if I am wrong but by ‘methylmercury’ did you not intend ‘ethylmercury’?

    Yes – thanks, corrected.

  12. RickKon 28 Jan 2009 at 4:29 pm

    Can’t we please get the language right in this debate. In the abortion debate, it is:

    pro-life vs pro-choice

    So shouldn’t the vaccine debate be:

    pro-vaccine vs pro-disease

  13. Watcheron 28 Jan 2009 at 11:20 pm

    There’s one problem with your suggestion, it’s not PC enough for both sides to be happy with. Each side has to be the “good” side with the name that means what stands for. One of the reasons why i hate PC …

    Also @ Perky Skeptic:

    Glad to see you put in the time to research the topic and, more importantly, not turn a blind eye to the research when it challenged your ideals. :)

  14. HHCon 29 Jan 2009 at 5:16 pm

    Click on the free abtract for this study on the Pediatrics page to read more.

  15. HHCon 29 Jan 2009 at 5:18 pm

    That’s Abstract!

  16. Perky Skepticon 29 Jan 2009 at 11:04 pm

    @Watcher– yay, facts! :)

  17. PEOPLEon 31 Jan 2009 at 2:00 am

    one can say vaccines are safe as many time as they want,but the fact remains the same ,we the parents know what we know,and that is vaccines cause autism and many other diseases .

    the profit wagons days are numbered and the bogus study’s can study as much as they want but the thruth is comming forward.

    i personaly know kids that had weak immune system that have been vaccinated against measles and ended up with chronic measles

    so all you non parents who are defending vaccines without knowing what it does to other peoples kids ,i have one thing to say to you.. SHAME ON YOU.

    WORRIED
    MOTHER

  18. son 31 Jan 2009 at 1:29 pm

    Do notice the caveat at the end of the report:
    “Our analysis included only healthy children who were selected during enrollment in the original trial, and some families might have declined to participate in the present study because their children had cognitive developmental problems. This might have reduced the prevalence of adverse neuropsychological conditions and might have made potential differences hard to detect. The eligibility criteria of the original trial also limited the participation of low birth weight children, and only 55 children with birth weights of

  19. son 31 Jan 2009 at 1:31 pm

    Grr a less than sign cut off my text…, let’s try again

    Do notice the caveat at the end of the report:
    “Our analysis included only healthy children who were selected during enrollment in the original trial, and some families might have declined to participate in the present study because their children had cognitive developmental problems. This might have reduced the prevalence of adverse neuropsychological conditions and might have made potential differences hard to detect. The eligibility criteria of the original trial also limited the participation of low birth weight children, and only 55 children with birth weights of [less than] 2500 g underwent the neuropsychological evaluation (data not shown). Moreover, only 1% of children in this study received hepatitis B virus vaccine at birth.”

    That will hardly convince the other side even if the study autism rate is similar to the national italian estimates, which according to the EURISPES, are between 6-10/10000. Well in line with the study results of 1/1403 (approx 7/10000).

    It would of course have been more conclusive and convincing if the study had included a control group of children vaccinated with vaccines containing no mercury compunds (and preferrably born of mothers that had never had amalgam fillings — to make causality very very clear).

    /S

  20. daedalus2uon 31 Jan 2009 at 2:12 pm

    You should check out the Faroe Island studies in mercury in cord blood (~1000 consecutive births). The same cohort was also tested for autism. In the 1404 child cohort that included the 1000 tested for mercury at birth, there were 5 cases of ASDs, 2 of autism and 3 of Asperger’s.

    In that 1404 child cohort there were at least 249 with cord blood mercury levels above 200 nM/L and at least 746 with cord blood mercury levels above 65 nM/L.

  21. HHCon 01 Feb 2009 at 12:57 am

    s, The disclaimer for this neuropsychological research is standard. Research is presented as tentative. Researchers are expected to be humble when they write science papers. This study has significant research results which will be built upon with future research designs. The conclusive results become a body of knowledge which we depend on when we go to court.

    Some parents with autism have great anger and shame based on their failure to produce perfect offspring. They have mental health problems which need community intervention.

  22. son 01 Feb 2009 at 9:52 pm

    HCC: Uhm my point was rather that such occasional disclaimers, combined with the absence of a “substance-naive” control group are the reason that the vaccine debate is stuck (just check the comments about the study on some of the autism sites), despite e.g. such facts as the above mentioned study happening to be in line with the supposed italian prevalence.

    D: Just localized the web sites of the anemone and chef projects–lot’s of studies. And in the end not what you referred to. I assume you wanted me to look at the “Autism in the Faroe Islands. An Epidemiological Study” by Ellefsen et al. showing autism rates of about 70/10000 as I noticed you earlier have discussed in another forum. Still not a naive pop though ;-)

    /S

  23. daedalus2uon 01 Feb 2009 at 10:09 pm

    My point wasn’t about a naïve group, it was about a very highly exposed group without an excess of autism. The mercury levels in the Faroe Island group are gigantic compared to any autistic group that has ever been suggested to have an excess of mercury via any actual measurement.

    This is the mercury reference.

    http://www.ehponline.org/docs/2005/7842/abstract.html

  24. HHCon 02 Feb 2009 at 12:50 am

    s, Your hypothetical control group will have to wait until a future study.

  25. son 02 Feb 2009 at 10:52 am

    yes :-)

  26. RealityBasedon 08 Feb 2009 at 1:11 pm

    Worried Mother states –

    “one can say vaccines are safe as many time as they want,but the fact remains the same ,we the parents know what we know,and that is vaccines cause autism and many other diseases .
    “the profit wagons days are numbered and the bogus study’s can study as much as they want but the thruth is comming forward.”

    Couple of questions –

    1: HOW do you “know” that “vaccines cause autism and many other diseases”? What evidence are you basing your conclusions on?

    2: If you’re really genuinely concerned about autism, then don’t you feel ashamed of yourself for wasting so much valuable time and energy barking up the wrong tree?

    Vaccines don’t cause autism – but clearly something else (probably many things) does. By fixating on something that doesn’t cause autism and blindly believing that it does in the face of all evidence to the contrary, you’re not just hurting your own kids and the children of everyone else who irrationally decides not to vaccinate out of a baseless fear of developing autism – you’re also hurting all the kids who actually do have autism or are at risk of developing it, because you’re diverting time and energy and attention away from research into real causes and real cures.

    To repeat:

    By insisting that vaccines cause autism and other diseases, you’re hurting children and putting their health at risk.

    For that you should be deeply, deeply, deeply ashamed of yourself.

  27. PEOPLEon 20 Feb 2009 at 12:12 am

    RealityBased:

    FYI.
    .This blog is not only for people who have same ideology like you do, its also for those of us who disagree.

    .I know vaccines cause autism, because of what happened to my child right after the vaccines, and when I say right after I mean right after three hours, so please before you make your comments educate your self on the issue.

    .Vaccines may prevent some diseases ,but they also do harm which in my Childs case was proven.

    “Vaccines don’t cause autism – but clearly something else ”

    .If you can’t tell me what causes autism ,then you can’t tell me vaccines don’t cause autism.

    .There is one thing if you are narrow minded and want to believe one side of the issue, but you should know there is other studies that says otherwise.

    Ps.Dr.Steven Novella should have people from both sides on this blog,because its really getting boring here.

    worried
    mother

  28. brittalynn0918on 20 Apr 2009 at 11:37 am

    i wholeheartedly agree with you # PEOPLE. If these people cared enough to research just a small bit they would realize that the facts are alarming!!

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