Between 1900 and 1930, companies packaged vaccines almost exclusively in multi-dose vials; with a typical vial containing ten doses. Doctors might keep such a multi-dose vial in their refrigerator for months at a time. However, this practice carried a significant risk of bacterial contamination upon reuse and there were several deadly incidents. In response, thimerosal was developed in 1928. It is a preservative that is just under 50% ethyl mercury and is more effective at reducing bacterial contamination than any other preservative developed to date. Thimerosal allows vaccines to be administered from multi-dose vials with a drastically lower risk of becoming contaminated. This is a much less expensive way of delivering vaccines than single dose vials and allows the cost of vaccination to be reduced dramatically.

Thimerosal has been intensively targeted in the last seven or so years by various groups who believe it (and in particular the ethyl mercury that is its main active ingredient) is partially or totally responsible for the so-called autism ‘epidemic’ in the western world in the last 20 or 30 years. The thimerosal theory of autism has been the most pervasive vaccine-based autism theory in the United States, contrasting with the MMR (measles-mumps-rubella) vaccine theory of autism that has been hugely prevalent in the United Kingdom. Similar to the MMR theory of autism, the thimerosal theory has never had any significant scientific support. And, in the last few years, the scientific evidence against the thimerosal theory has reached a level that is largely unassailable.