A new paper published in PNAS goes where many studies have gone over the last ten years: it shows that the administration of vaccines does not result in the development of autism-like behavior or brain malfunction. But it does so in dramatic fashion, by giving vaccines to young primates and tracking their development.

This paper, along with many others on this topic over the last several years, aims to correct misconceptions among the public (and presidential candidates) that vaccination is linked to autism. These were fueled by a 1998 publication (since retracted) Lancet paper by Andrew Wakefield. A sector of the public consciousness has been captured by the suggestion that there may be a link between the measles, mumps, rubella vaccine and Autism Spectrum Disorders.

The aforementioned paper was retracted due to questionable methodology and a financial conflict of interest. But it linked use of the vaccine preservative thimerosal to the development of autism in a very small cohort of participants. After the paper was published but before its problems became apparent, this preservative was removed from vaccines delivered in childhood as a precautionary measure to reassure parents.

Since then, several large-scale, methodologically sound studies have failed to link autism to vaccination. Yet there remains a strong anti-vaccination movement in the United States. Despite the risks of not vaccinating, parents continue to fear vaccination and are opting out of the recommend schedule. Recent upticks in our failure to vaccinate have contributed to outbreaks of preventable diseases, such as the 2015 Disneyland Measles outbreak.

The authors of this study examined 79 infant rhesus macaques. These monkeys were either placed into a control group, where they were given saline injections, or they were given an injection of the tri-valent MMR vaccine. This vaccine, which contained the preservative thimerosal, was very similar to the vaccine that Wakefield initially connected to autism in his 1998 paper.

After injections, the monkeys were tested for autism like nonsocial behavior, fear behaviors, and repetitive activities. The examined behaviors of the monkeys that received the MMR vaccine were not significantly different from those that received the saline injection. The researchers to conclude that the monkey’s propensity for autism-like behaviors was not affected by their exposure to the MMR vaccine with thimerosal.

The authors also examined the monkey’s brains post-injection, to look for brain abnormalities analogous to those that have been found in human patients with autism spectrum disorder. These included abnormalities in the cerebellum, the amygdala, and the hippocampus. The researchers found no significant differences between the brains of the vaccinated monkeys and the control monkeys for any of these regions. Again, there was nothing to implicate the MMR vaccine with thimerosal in altering neural development.

The data from this study add to a large body of literature supporting the conclusion that childhood vaccinations are not linked to autism like pathology or behavioral changes. Though there are many studies supporting these conclusions, it can be challenging to convince parents who are already strongly driven by fear of vaccination. It may take further events that display the dangers of forgoing vaccination for this fear to lose its grip on the minds of the public.

PNAS, 2015. DOI: 10.1073/pnas.1500968112 (About DOIs).