NY Times Magazine tells the story of how one the top USA vaccine advocates, Neal Halsey, became a critic when he discovered that thimerosal (a mercury compound) in vaccines could be causing an epidemic of autism.

And then suddenly in June 1999, during a visit to the Food and Drug Administration, a squall appeared on the horizon of Halsey's confidence. Halsey attended a meeting to discuss thimerosal, a mercury-containing preservative that at the time was being used in several vaccines -- including the hepatitis B shot that Halsey had fought so hard to have administered to American babies. By the time the dust kicked up in that meeting had settled, Halsey would be forced to reckon with the hypothesis that thimerosal had damaged the brains of immunized infants and may have contributed to the unexplained explosion in the number of cases of autism being diagnosed in children.

Making matters worse, the latest science on mercury damage suggested that even small amounts of organic mercury could do harm to the fetal brain. Some of the federal safety guidelines on mercury were relaxed in the 90's, even as the amount of mercury that children received in vaccines increased. The more Halsey learned about these mercury studies, the more he worried.

The article explain how others in the vaccine establishment wanted to cover up the mercury problem. Paul Offit, a vaccinologist at the Children's Hospital of Philadelphia, said: ''In some instances I think full disclosure can be harmful''.