New research finds no evidence that thimerosal-containing vaccines cause negative behaviors or result in neuropathology in infant primates, according to a study (PDF) published today in PNAS, the Proceedings of the National Academy of Sciences.

In the study, infant rhesus macaques received several pediatric vaccines containing thimerosal, a mercury-based preservative, in a schedule similar to that given to infants in the 1990s. Other animals received just the measles-mumps-rubella (MMR) vaccine, which does not contain thimerosal, or an expanded vaccine schedule similar to that recommended for U.S. infants today. Control animals received a saline injection. 

Regardless of vaccination status, all animals developed normal social behaviors; the administration of vaccines to rhesus macaques did not result in neuropathological abnormalities or aberrant behaviors such as those often observed in autism.

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Study data indicates that vaccines in infant primates did not cause behaviors or brain changes associated with autism.

picture of the word "autism" on top of white interlocking puzzle pieces

Cellular analysis of the cerebellum, amygdala and hippocampus – three brain regions known to be altered in autism – was similar in vaccinated and unvaccinated animals. 

"This comprehensive study included many physiological measures and behavioral measures. Fundamentally the vaccines had no ill effects," said Gene Sackett, UW professor emeritus of psychology and director of the lab work at the Washington National Primate Research Center. "To the extent that macaques mirror human physiology, I think this bears out what most people have known: These vaccines are safe."

Laura Hewitson of The Johnson Center for Child Health and Development, a principle investigator, said the study was designed to compare the safety of different vaccination schedules, including the schedule from the 1990s, when thimerosal was used as a preservative in multidose vaccine preparations.

The research also involved the UW's Center on Human Development and Disability; the University of Texas Southwestern; and Texas A&M Health Science Center & Central Texas Veterans Health Care System.