Thimerosal in vaccines did not cause an autism epidemic

There have been two main theories linking vaccines to an “epidemic” of autism. Both theories have been studied. Both have been heard in the courts. Neither theory had a sound scientific basis and epidemiological data has shown that neither theories explained the increase in autism prevalence in the last 20 years.

First it was proposed that the MMR vaccine resulted in persistent measles infections that lodged in the intestines of children leading to “leaky guts” and that harmful substances were leaked into the blood, traveled to the brain and resulted in autism symptoms. This was proposed by Dr. Andrew Wakefield and has since been shown in epidemiological and other studies to be unsound. (This theory morphed for the Omnibus Autism Proceeding, the vaccine court. The argument there was that the measles virus itself traveled to the brain. Again, it is not supported by epidemiological data and is not scientifically sound).

The second theory was that mercury in vaccines from a compound called thimerosal caused autism. In that theory, it was proposed that autism symptoms were similar to mercury poisoning (autism was a “novel” form of mercury poisoning). This theory was not scientifically sound as autism symptoms are not like mercury poisoning. Previous epidemiological studies have also shown thimerosal was not behind the rising numbers of people diagnosed with autism.

In 2007 there was a study which looked at 1,000 kids aged 7-10 to see if various neurological symptoms were more prevalent in those who received higher exposures to thimerosal. Orac at Respectful Insolence blogged it and Kev posted that piece here on LeftBrainRightBrain as well. That study showed indications that in some measures children may perform more poorly with thimerosal exposure. It also showed that in some measures children may perform better with thimerosal exposure. This mixed result is (a) not very strong in either direction and (b) not very surprising when you look at a lot of different measures at the same time. Chance will result in some measures positive, some negative.

The 2007 study was published in the New England Journal of Medicine as Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years, by Thompson, et al.. (Thompson (2007))

What was missing in that report was a direct study of autism. Given the numbers of children (1,047) selected, there would only be about 10 kids with ASD expected in the group. This is too few for a strong conclusion on autism. At the time of that study it was noted that another study would follow concentrating on autism alone.

That study has just been published in the journal Pediatrics as Prenatal and Infant Exposure to Thimerosal From Vaccines and Immunoglobulins and Risk of Autism. They studied “256 children with ASD and 752 controls matched by birth year, gender, and [managed care organizations]”. I will give some details here. I expect the treatment on the Science Based Medicine and Steven Novela’s Neurologica blogs to cover the science thoroughly should you wish more detail.

Short answer: thimerosal exposure doesn’t cause an increased risk of autism. Neither thimerosal from vaccines given to the children nor thimerosal from products like Rhogam are behind the increase in autism prevalence we have seen.

It is worth noting that the authors looked at autism with and without regression.

Here is the abstract:

OBJECTIVE: Exposure to thimerosal, a mercury-containing preservative that is used in vaccines and immunoglobulin preparations, has been hypothesized to be associated with increased risk of autism spectrum disorder (ASD). This study was designed to examine relationships between prenatal and infant ethylmercury exposure from thimerosal containing vaccines and/or immunoglobulin preparations and ASD and 2 ASD subcategories: autistic disorder (AD) and ASD with regression.

METHODS: A case-control study was conducted in 3 managed care organizations (MCOs) of 256 children with ASD and 752 controls matched by birth year, gender, and MCO. ASD diagnoses were validated through standardized in-person evaluations. Exposure to thimerosal in vaccines and immunoglobulin preparations was determined from electronic immunization registries, medical charts, and parent interviews. Information on potential confounding factors was obtained from the interviews and medical charts. We used conditional logistic regression to assess associations between ASD, AD, and ASD with regression and exposure to ethylmercury during prenatal, birth-to-1 month, birthto-7-month, and birth-to-20-month periods.

RESULTS: There were no findings of increased risk for any of the 3 ASD outcomes. The adjusted odds ratios (95% confidence intervals) for ASD associated with a 2-SD increase in ethylmercury exposure were 1.12 (0.83–1.51) for prenatal exposure, 0.88 (0.62–1.26) for exposure from birth to 1 month, 0.60 (0.36–0.99) for exposure from birth to 7 months, and 0.60 (0.32– 0.97) for exposure from birth to 20 months.

CONCLUSIONS: In our study of MCO members, prenatal and early-life exposure to ethylmercury from thimerosal-containing vaccines and immunoglobulin preparations was not related to increased risk ASDs. Pediatrics 2010;126:656–664

My guess is that there will be much discussion of the methods on many websites. For now, here are the data from Table 2 and Table 3.

Table 2 (click to enlarge)

Table 3 (click to enlarge)

As with Thompson (2007) the authors will make longer reports available on their website and will allow access to the data.

This study is not the first of its kind. Here are a few of the large studies which have shown a lack of association between thimerosal exposure and autism in the past.

Thimerosal and the Occurrence of Autism: Negative Ecological Evidence From Danish Population-Based Data

Safety of Thimerosal-Containing Vaccines: A Two-Phased Study of Computerized Health Maintenance Organization Database

Thimerosal Exposure in Infants and Developmental Disorders: A Prospective Cohort Study in the United Kingdom Does Not Support a Causal Association

Autism and Thimerosal-Containing Vaccines Lack of Consistent Evidence for an Association

There are more.

One question is whether this will finally quiet those claiming an autism epidemic caused by mercury in vaccines. Unfortunately, I sincerely doubt it. This study included Sallie Bernard of SafeMinds in the acknowledgments. Ms. Bernard was also involved in the Thompson study of 2007. At that time she was listed as a “dissenting” member of the team. She submitted a letter to the NEJM discussing the reasons for her dissention, Perhaps the lack of the word “dissenting” this time is a good sign. I’ll wait and see.

The main question is how much impact this will have on the next generation of families with autistic children. I can’t but wonder that the age of the mercury hypothesis has seen its peak. Not only in research but in general acceptance.