Mercury and Autism: A briefing

There is no doubt that the heavy metal mercury (Hg) is one of the most toxic substances on earth. The role of mercury as a major possible cause of autism has come to light only in the last 18 months after it was realised that the amount of mercury preservative in many vaccines was well in excess of recommended safety standards (Rimland 2001).

The concern has been highest in the USA where use of mercury is more widespread and indeed where the number of vaccines received by children is higher at earlier ages and often multiple vaccines are applied in a single day. Concerns have been expressed by parents following the revelation that the 75 symptoms of mercury poisoning parallel those for autism (Redwood 2000).

Mercury is used as a preservative under the name thiomersal (also known as thimerosal) in vaccines such as those against Hepatitis B and DPT (diphtheria, pertussis and tetanus) but is not used in live, attenuated vaccines such as MMR although some claims suggest that there may be trace elements found even in these products. In the UK childhood vaccines which definitely contain mercury include DPT and the DT (diphtheria and tetanus) preschool booster.

Mercury poisoning is often mistaken for a psychiatric disorder, especially when mercury exposure is not suspected (Bernard et al 2000). Exposure can also occur through maternal consumption of contaminated fish, dental amalgams, its use in antibiotics and other medications, and the anti D vaccination given to mothers during pregnancy or just after childbirth, which reduces the risk of RhD alloimmunisation in Rhesus negative women who have given birth to a Rhesus positive infant. There are concerns that mercury may be one element in a multiple assault on the system, which could trigger autism spectrum conditions. These are speculative at present.

The causes of autism are currently, for the most part, unknown. 90% of cases are idiopathic (of unknown cause). Present knowledge suggests that autism is a strongly, but not wholly, genetic condition (Lamb et al 2000). It is thought to be a multigenic (more than one gene) condition (Turner, Barnby and Bailey 2000). It is likely that environmental factors also play a part in autism. What these environmental factors are is unclear. For this reason a number of candidates have been suggested, including the MMR vaccine and, more recently, mercury. Whilst these might be plausible suggestions there is, as yet, no sound scientific evidence to link autism with mercury. However, Geier and Geier (2003) suggest that there is strong epidemiological evidence for a link between increasing mercury from thiomersal-containing vaccines and neurodevelopmental disorders and heart disease. They argue for the immediate and complete removal of thiomersal from all childhood vaccines.

The European Agency for the Evaluation of Medicinal Products (EMEA) in its position statement (EMEA 2000) stresses the conclusions of the evaluation by the Centers for Disease Control and Prevention (CDC) in the USA into whether there is any epidemiological association between thiomersal-containing vaccines and specific neurological disorders in children. The evaluation found the studies made thus far to be inconclusive and lacking in sound scientific information. Further studies were recommended.

Meanwhile steps were being taken to eliminate organo-mercurials used as preservatives in the finished vaccine products. The EMEA's Committee for Proprietary Medicinal Products (CPMP) recommended a step-wise approach to the removal of organo-mercurials in July 1999 provided that the benefit/risk analysis remains unchanged. The USA's CDC has recommended a similar approach. This means that existing stocks of organo-mercurial containing vaccines are being used up by manufacturers. The explanation given for this is that it takes some while to make several changes to the manufacturing process.

The CPMP recommends continuing to vaccinate with existing vaccines including those containing thiomersal as they continue to offer benefits over and above the risks, if any, of exposure to mercury from these products. It recommends also that as a precautionary measure for vaccinating infants and toddlers that it would be prudent to use vaccines without mercurial-containing preservatives. The available thiomersal-free vaccines are recommended for use in newborn children.

Politically it may be seen to be unwise to raise concerns about safety, even at a precautionary level, but then to continue to use those vaccines. There is some anecdotal evidence that parents who wish to choose thiomersal-free vaccines for their children are not able always to achieve this.

The National Autistic Society (NAS) has supported parents' rights to choice for their children. The use of thiomersal-free vaccines is recommended and moves are underway towards the elimination of organo-mercurials. The NAS supports this process and urges all speed be made towards ensuring that these products are not used within the immunisation programme. Where mercury-free vaccines are available they should be used across the board as a precautionary measure although there is no persuasive evidence at present linking mercury with autism. It would also seem sensible to ensure that the constituents of vaccines and medications should always be clear both to health professionals and consumers.

In August 2004 the NHS announced that a new combined vaccine for diphtheria, tetanus, pertussis, Hib and polio was to be introduced from September 2004. The new vaccine does not contain thiomersal, which its predecessor did. The introduction of this thiomersal-free vaccine, according to the NHS press release, dated 09/08/2004, "meets the internationally agreed aim of reducing the exposure of children to mercury if it can be avoided".