Thimerosal, a mercury-containing preservative in vaccines, causes neurological damage and autism.
Thimerosal became a focal point of vaccine anxiety in the 1990s, even though the mercury form in thimerosal (ethylmercury) is metabolized and excreted within days, unlike the toxic form (methylmercury) that bioaccumulates in fish. The fear stemmed from legitimate concern about methylmercury exposure, but applied to the wrong compound. Thimerosal was phased out of most childhood vaccines by 2001 as a precautionary measure, not because it was proven harmful, no causal link to autism or neurotoxicity was ever established. Multiple large-scale studies comparing vaccinated and unvaccinated children, including a 2014 meta-analysis covering millions of children, found no association between thimerosal exposure and autism or developmental disorders. Blood mercury levels in vaccinated children remained well below toxicity thresholds. The removal of thimerosal from vaccines was ultimately good public health policy, but its removal paradoxically reinforced the false idea that it had been dangerous.