In 1999, the US Institute of Medicine’s Immunization Safety Review Committee released a report examining the effect of the mercury preservative thimerosal, commonly contained in certain vaccines, notably, the triple DPT jab and DT booster in the UK, and most vaccines in the US. The IOM concluded that there was not enough evidence to prove one way or the other whether mercury can cause learning disabilities in children. It did, however, find enough evidence that mercury can damage the brain to recommend that all mercury preservatives be removed from vaccines and over-the-counter products. The US Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) ordered drug companies to remove mercury from childhood vaccines that year.

Also, according to the law firm of Walters & Kraus, the Centers for Disease Control and Prevention (CDC) assembled a confidential report, still unreleased, showing that exposure to significant amounts of mercury during the first months of life can significantly increase a child’s risk of developing autism. The report - obtained by SAFEMINDS, the American advocacy group - discovered a 2.48 times increased risk of autism in children exposed to more than 62.5 mcg of mercury before the age of three months. The law firm, representing parents in at least 25 states, has filed lawsuits attempting to force drug companies to investigate the link between mercury in vaccines and developmental disorders.

Animal studies, among the few to examine the effect of agents like thimerosal on living things, have found that seven of the most commonly used substances can produce tumours (Clin Toxicol, 1971; 4: 185). Another study found that patients given immunoglobulin preserved with thimerosal, as found in vaccines, had raised mercury levels (BMJ, 1979; ii: 12). Other studies have shown that thimerosal has a negative effect on white blood cells (Am J Public Health, 1940; 30: 129).

The question of hypersensitivity is also important. One study showed that more than a third of allergic patients undergoing allergy desensitisation with shots containing thimerosal developed hypersensitivity to the mercury salt (Contact Derm, 1989; 20: 173-6). This ‘hypersensitivity’ has been shown to be a direct effect of initial exposure to the salt via vaccination. Indeed, high sensitivity to thimerosal is due to previous exposure to the agent in vaccinations (Contact Derm, 1980; 6: 241-5). In animals, mercury salts can cause immune suppression (Toxicol Appl Pharmacol, 1983; 68: 218-28), although this may not apply to humans.