Via Autism Diva I learn of this study just published in the journal Pediatrics. The objective of the study was to estimate the prevalence of pervasive developmental disorder (PDD), which includes autism, and compare this with cumulative exposure to both thimerosal and MMR vaccines. The authors surveyed 28,000 children born from 1987 to 1998. Children with PDD were identified by a special needs team.
What they actually found was that children who had had no exposure to thimerosal actually had a higher degree of PDD. No association to PDD was found with the MMR vaccine. From the abstract (all quotes are with my bold):
The prevalence of pervasive developmental disorder in thimerosal free birth cohorts was significantly higher than that in thimerosal-exposed cohorts (82.7 of 10 000 vs 59.5 of 10 000). Using logistic regression models of the prevalence data, we found no significant effect of thimerosal exposure used either as a continuous or a categorical variable. Thus, thimerosal exposure was unrelated to the increasing trend in pervasive developmental disorder prevalence.
The authors conclude that the increased prevalence of PDD is due to broadening diagnostic criteria and increased awareness. If you remember, this is the explanation that Bobby Kennedy called “a theory that seems questionable at best, given that most of the new cases of autism are clustered within a single generation of children”. Well, it’s hard to see what other conclusion could be drawn from this study. From the paper again:
During the 12 years encompassed by our study, thimerosal exposure before age 2 of each birth cohort changed several times and ranged from nil to a high value of 225 μg. This provided a unique opportunity to test the relationship of ethylmercury exposure with rates of PDDs, free of a known problem of vaccine safety studies when high rates of exposure in populations, and therefore low variability in exposure, constrain the data and limit the opportunity to detect effects.63 No association between thimerosal levels treated either continuously or categorically with PDD rates could be found in our study. In fact, it was remarkable that the PDD rates were at their highest value in birth cohorts that were thimerosal free, providing a clear and convincing message on the lack of an association. The results were robust and held true when various analyses were conducted to evaluate the potential impact of misclassification on exposure and diagnosis. Within each period of medium, high, or nil exposure, the same trend toward a steady increase in PDD rate was observed, demonstrating total independence of the 2 variables.
The authors also address one of the mercury-causes-autism lobby’s criticisms of other studies that have also shown no link, namely that these other studies were in countries with lower exposure to thimerosal or lower rates of PDD:
Previous negative studies, especially those conducted in European countries, have sometimes been criticized on the account that either the rates of PDDs were not as high as those in North America, that the cumulative exposure to thimerosal was much lower than that attained in the United States in the 1990s, or both. This study avoids both pitfalls and is, therefore, very informative for the North American public. In addition, the rate of exposure varied from nil to very high levels of vaccine-derived ethylmercury, allowing us to test for effects along the full range of exposure and to detect possible threshold effects as well. All of the results were negative.
Of course, don’t expect this study to convince the religious mercury-causes-autism groups. From a (pretty good) BBC article on the study we find:
Jackie Fletcher, from campaign group Jabs, a support network for parents who believe their children have been damaged by vaccines, said the study still did not prove there was not a link.
Well, proving a universal negative is always difficult, but with one more seemingly well run study finding no link when you would expect to find one if it were there, haven’t we passed the point when it becomes perverse to insist there is a link?
Edited to add:
Two more excellent reads today on this subject. Respectful Insolence has a more detailed write-up about this study, along with an examination of some of the usual smear tactics used by the mercury-causes-autism group SafeMinds. Left Brain/Right Brain has a further perspective on SafeMinds hypocritical ad hominems, and insight on where they might be headed in their future mercury claims.