In the late 1990s, most vaccines used thimerosal as a preservative. In addition, there were a number of new vaccines containing thimerosal that were added to the recommended vaccine schedule for children in the first year of life. More vaccines with thimerosal meant each child was receiving more ethylmercury in the first twelve months of life. In the end, the potential amount of ethylmercury in the recommended vaccines exceeded the EPA-recommended maximum dose of mercury for infants.
While there was no proof that the ethylmercury in thimerosal was toxic, there was clear evidence that a related chemical, methylmercury, was harmful. The most famous example of methylmercury poisoning occurred in Minamata Bay, Japan, where an industrial company dumped mercury into the bay for decades. The methylmercury became concentrated in fish and shellfish, and subsequently affected people who ate the toxic fish. The results were catastrophic, with hundreds of people suffering from nervous-system symptoms such as an abnormal gait, abnormal speech, comas, and even death. Methylmercury also concentrates in the fetal bloodstream and leads to congenital methylmercury poisoning. This leads to birth defects and cerebral palsy in affected children.
Given the known risk of methylmercury poisoning, in July 1999, the American Acadeour of Pediatrics (AAP) and the U.S. Public Health Service (USPHS) recommended removing ethylmercury from vaccines as well as deferring the birth dose of the hepatitis B vaccine until the thimerosal had been removed. Since 2001, all vaccines that can be given to young children are thimerosal-free. The only exception is one version of the flu vaccine for children that still contains a trace of thimerosal, but less than 1 percent of the previous dose. However, there are other thimerosal-free flu vaccines available for children. Some vaccines meant for older children and adults still use thimerosal as a preservative. The supposed rationale for the remaining thimerosal is that older children and adults weigh more and are thus better able to tolerate larger amounts of ethylmercury.