A few years back (1999, to be precise), there was a big expose that linked autism to childhood vaccinations (specifically ones with the chemical thiomersal, an organo-mercury compound). The original work was purely correlative, but many parents took this danger to heart, and refused to give their child any vaccinations at all. There has, since, been a slow and steady move to reject the hypothesis that thiomersal is causally linked to autism, since very little actual evidence has been uncovered to support that hypothesis and several studies have shown no real link between the two.
Today, I came across a poster (Shrikanth et al.*) that examined the thiomersal-vaccine issue from a new perspective: a macaque monkey model. They administered the same schedule of vaccines that human babies received during the period when people noticed a correlation, but scaled so as to reflect the monkey’s faster developmental trajectory (~4x faster). Interestingly enough, Shrikanth and colleagues found several neurological similarities between humans with autism and monkeys exposed to the thiomersal-containing vaccines: both had a decrease in the number of Purkinje cells (for neuroscience folk, GABAergic neurons in the cerebellum), which may be involved in the loss of motor control that 60-80% of autistics suffer from (http://www.annualreviews.org/doi/abs/10.1146/annurev.med.60.053107.121225); an increase in the size of the amygdala; and a decrease in the size of C1 neurons in the hippocampus. The study isn’t complete yet – they have yet to do a few more tests to rule out the effects of some other substances in vaccines that might potentially have an effect. This program of research may well conclude that thiomersal has little to no impact on a child’s risk for autism, but these initial findings give us a clue that there might be more truth to the vaccination hypothesis than the current consensus allows.
This study is one very good example of a larger process at work that deserves a moment of note: the scientific process is really kicking in. In this example, we (i.e. the scientific community) found a correlation that surprised us, turned it into a probing hypothesis, realized that it was not being validated by the research we had, and are now coming together with a disciplined program of study to explain the correlation from multiple perspectives. Good job, science. You have served us well.
*The study is still in preparation, so there’s no full paper on it. At the talk, it was titled, “Influence of pediatric vaccines on CNS development in the rhesus macaque: Cerebellar Purkinje cells.” The abstract is on page 669 of this page: http://www.sfn.org/am2011/pdf/prelim/SAT_Poster_PM_v2.pdf