During the most recent Republican presidential debate, frontrunner Donald Trump once again dragged out the still widespread myth that vaccines cause autism. This dangerous fiction was debunked as early as 2002 by the New England Journal of Medicine and has been consistently contradicted by research ever since. As a result, anti-vaxxers changed strategy: Instead of blaming thimerosal for causing autism, they now focus on the vaccine schedule itself, essentially claiming that too many shots in too short of a timespan overwhelms a child's immune system.

That is nonsense. The number of antigens (i.e., molecules that trigger an antibody response) contained within vaccines has decreased dramatically over the past several decades. In the Genetic Expert News Service, Emory University infectious disease professor Dr. Walter Orenstein says that the total number of antigens in all vaccines combined is about 150, which is practically nothing compared to the roughly 2,000-6,000 antigens children face every single day. By crawling around on the floor and sticking their hands in their mouths, children are "vaccinating" themselves all day long.

Despite the lack of scientific logic to the anti-vaxxers' argument, a team of researchers decided to address the issue of vaccine schedules head-on. Their results are reported in the journal PNAS.

The team divided 79 rhesus macaque monkeys into six experimental groups, each containing 12-16 animals. They then administered various combinations of vaccines to the animals in those groups. [Note that some vaccines contain EtHg, a metabolic derivative of thimerosal, while others do not. Also note that the vaccines used are the same as or similar to those used to vaccinate children (PDF).] The list of vaccines is shown below:

The groups were divided as follows:

Control: Saline placebos only

1990s Pediatric: The 1990s vaccine regimen for children

1990s Primate: A four-fold acceleration of the 1990s regimen

TCVs: Only thimerosal-containing vaccines, but MMR replaced with saline placebo

MMR: MMR only and saline placebos for other vaccines

2008: The 2008 vaccine regimen for children (which is similar to that used today)

After all shots were administered, the macaques' behavior was assessed and their brains were examined for signs of autism. What did they find?

Nothing. Absolutely nothing. There were neither significant differences in brain structure nor significant differences in the negative behaviors associated with autism. In other words, all the monkeys developed normally.

Critics will argue that this experiment was performed in macaques, not humans, and is therefore unreliable. Of course, it is unethical to give children placebos in place of vaccines (not to mention cutting their brains open), so this experiment cannot be performed in humans.

Because the coffin of the vaccine-autism myth has already been nailed shut, this study really should represent the flowers placed on top of the grave. Sadly, it probably won't make a dime's bit of difference to the anti-vaxxers who continue to see a conspiracy behind every corner.

Source: Bharathi S. Gadad et al. "Administration of thimerosal-containing vaccines to infant rhesus macaques does not result in autism-like behavior or neuropathology." PNAS. Published online before print: 28-Sept-2015. doi: 10.1073/pnas.1500968112

(AP photo)