evidence: NARRATOR: Vaccine critics questioned not only MMR but the many ingredients in vaccines. These historically have included aluminum, formaldehyde, formalin, yeast protein and a mercury-based preservative called thimerosal used in multi-dose vaccines. One very influential critic is environmental lawyer Robert Kennedy, Jr. [...] Dr. ERIC FOMBONNE: So the mercury hypothesis had more plausibility than that for MMR because of the nature of mercury, which is known to be a neurotoxicant. But mercury poisoning does not resemble autism at all. The fact that mercury is a toxicant is not sufficient to claim that the mercury contained in vaccines can lead to autism. So it was critical therefore to follow this hypothesis with properly conducted epidemiological studies. [...] NARRATOR: Other studies in the U.S., the U.K. and Canada also found no association between autism and thimerosal. In 2004, the Institute of Medicine carried out its own analysis of the research and concluded, "The body of epidemiological evidence favors rejection of a causal relationship between the MMR vaccine and autism and also between thimerosal-containing vaccines and autism."