While every major health organization, researcher, and certified physician can agree that vaccines don’t cause autism, the anti-vaxxers out there remain stalwart that somehow, the link must still exist, and it’s simply due to incorrect data that it hasn’t been proven yet. To most of us, it seems ludicrous that they cling to this notion, especially after scores of scientific evidence have proven that vaccines are entirely safe and do not cause autism.

Now, it appears that the efforts of a certain group of anti-vaxxers have backfired, Newsweek reported. The anti-vaccination group, known as SafeMinds, funded a recent study published in the Proceedings of the National Academy of Sciences that examined the link between autism and vaccines over the course of six years — and the study found no link at all, perhaps to their frustration.

The researchers examined 79 infant monkeys divided into six groups; two of the groups were given vaccines containing thimerosal — an antiseptic and antifungal component that was removed from vaccines recently. Two other groups were given the measles, mumps, and rubella (MMR) vaccine — which is often cited by anti-vaxxers as a source of autism — and the last two groups were given a saline injection as a control.

Their conclusion was straightforward: Neither the thimerosal-containing vaccines nor the MMR vaccines changed the behavior of the monkeys, and they didn’t contribute at all to any shrinking of the hippocampus or other regions of the brain (which has, in the past, been associated with autism).

“No behavioral changes were observed in the vaccinated animals,” the authors write, “nor were there neuropathological changes in the cerebellum, hippocampus, or amygdala. This study does not support the hypothesis that thimerosal-containing vaccines and/or MMR vaccine play a role in the etiology of autism.”

Despite the crystal clear conclusion, SafeMinds continues to argue that the results are “controversial,” still unclear, and open to interpretation.

“SafeMinds has concerns about changes in the study design protocol and analysis that may have led to these contradictory results,” the group said in a statement. “We are in the process of collecting and reviewing additional information regarding this study.”

Fortunately, recent polls have shown that the numbers of warring anti-vaxxers are declining. But there’s still a lot of work to be done to convince those who still cling to the vaccine-autism link. To start, it’s important to remind them that vaccines are incredibly important to the health of all children; but perhaps we can also note that even when they fund their own studies, the science says it all.

Source: Gabad B, Li W, Yazdani U, Grady S, Johnson T, Hammond J. Administration of thimerosal-containing vaccines to infant rhesus macaques does not result in autism-like behavior or neuropathology. Proceedings of the National Academy of Sciences. 2015.