A number of vaccines, including those given to very young children, have contained thiomersal, a preservative that metabolizes into ethylmercury. It has been used in some influenza, DTP (diphtheria, tetanus and pertussis) vaccine formulations. Since 1997, use of thimerosal has been gradually diminishing in western industrialized countries after recommendations by medical authorities, but trace amounts of thimerosal remain in many vaccines and in some vaccines, thimerosal has not yet been phased out despite recommendations. Some states in the USA have enacted laws banning the use of thimerosal in childhood vaccines.

Thiomersal is very toxic by inhalation, ingestion, and in contact with skin (EC hazard symbol T+), with danger of cumulative effects. It is also very toxic to aquatic organisms, and may cause long-term adverse effects in aquatic environments (EC hazard symbol N).[5] In the body, it is metabolized or degraded to ethylmercury (C2H5Hg+) and thiosalicylate.[1]

Few studies of the toxicity of thiomersal in humans have been performed. Animal experiments suggest that thimerosal rapidly dissociates to release ethylmercury after injection, that the disposition patterns of mercury are similar to those after exposure to equivalent doses of ethylmercury chloride, and that the central nervous system and the kidneys are targets, with lack of motor coordination being a common sign. Similar signs and symptoms have been observed in accidental human poisonings. The mechanisms of toxic action are unknown.

Many parents, and some scientists and doctors, believe there is a connection between thiomersal and autism. More than 5000 U.S. families have filed claims alleging autism was caused by vaccines, most implicating thiomersal; most claims are still being adjudicated and no awards have been given.[10]

In the late 1990s, controversy over vaccines escalated in both the US and the United Kingdom when a study, published in the respected journal Lancet, by Dr. Andrew Wakefield suggested a possible link between bowel disorders, autism and the MMR vaccine, and urged further research.[1] His report garnered significant media attention, leading to a drop in the uptake of the MMR vaccine in the United Kingdom and some other countries.

"Up to 90% of the total decline in the death rate of children between 1860-1965 because of whooping cough, scarlet fever, diptheria, and measles occured before the introduction of immunisations and antibiotics."---Dr Archie Kalokerinos, M.D.