Thimerosal

As told: cumulative ethylmercury exposure from the expanded childhood schedule of the 1990s exceeded federal safety thresholds for methylmercury, and the resulting neurotoxicity explains the parallel rise in autism diagnoses. The precautionary removal of thimerosal in 1999 is presented as an admission.

A 1999 FDA review of cumulative mercury exposure across the schedule prompted the US Public Health Service and the American Academy of Pediatrics to ask manufacturers to remove thimerosal as a precaution, while stating no evidence of harm existed. The removal was widely read as confirmation. Robert F. Kennedy Jr's 2005 article Deadly Immunity, later retracted by Salon, brought the claim to a mass audience.

Thimerosal metabolises to ethylmercury, which clears from blood with a half-life of days; the safety thresholds it was compared against were derived for methylmercury, which bioaccumulates over months. The comparison was a category error, and that was the basis of the precautionary decision rather than of any finding. The decisive evidence is what happened afterwards: thimerosal was removed from all routine US childhood vaccines by 2001, and from Danish vaccines in 1992, and autism diagnoses continued to rise on the same trajectory in both countries. Nine large epidemiological studies across four countries have found no association. Thimerosal remains in some multi-dose influenza vials, where the alternative in much of the world is no vaccine at all.