Since the 1990s, thimerosal, a mercury-containing preservative used in some vaccines, antigens and immunoglobulins, has been a topic of debate around its safety and its possible link to autism. Studies in the past have gone both ways.

The new study, conducted by the University of Missouri-Columbia and published this month in the American Journal of Medical Genetics, investigated thimerosal exposure during pregnancies, via Rh immune globulin (RhIg), that resulted in the birth of a child subsequently diagnosed with autism. RhIg, which contains thimerosal as a preservative, is used to prevent haemolytic disease in pregnant Rh negative women.

The results showed that exposure to RhIg preserved with mercury-containing thimerosal before birth was no higher for children with autism.

Study author Judith Miles, who is the William S. Thomson endowed chair of autism and professor of paediatrics and pathology at the MU School of Medicine, said: "The study adds to the evidence that there is no casual association between thimerosal and childhood autism. We conclude that there is no indication that pregnancies resulting in children with autism were more likely to be complicated by Rh immune globulin/thimerosal exposure."

"We hope this report of no association between autism, Rh negativity and thimerosal exposure during pregnancy will offset some of the decreased compliance with immunization recommendations which is known to increase morbidity and mortality from childhood infectious diseases," she said.

Thimerosal contains about 50 per cent ethyl mercury and has been used in vaccines since the 1930s as a preservative to help prevent life-threatening bacterial contamination. It was developed into vaccines after 11 children died in 1928 after they were immunized against diphtheria with a vaccine which had become contaminated with living staphylococci.

Controversy is still rife regarding thimerosal and its connection with autism. Just last month a study was published in the Journal of Toxicology and Environmental Health, Part A, which stated that the study left little doubt there was a "direct causal link" between thimerosal exposure in vaccines and mercury poisoning diagnosed as an autism spectrum disorder.

The claim there was a connection between autism and thimerosal first began in the 1990s following a surge in the rates of the neurological disease. Since then, vaccine manufacturers have had claims against them and have even faced legal action. Merck & Co has been named in more than 4000 claims filed by parents claiming their children suffered autism or other neurodevelopmental disorders from mercury in vaccines.

In 1999 the US Food and Drug Administration (FDA) made its concerns about the effects of the preservative on child development public and made efforts to minimise the use of thimerosal in vaccines. By 2002 the preservative was removed from all routinely recommended childhood vaccines in the US.

In Europe and the UK, there is no equivalent ban in place but the European Agency for the Evaluation of Medicinal Products (EMEA) promotes the use of other preservatives where possible, such as 2-phenoxyethanol, phenol and benzethonium chloride, while applications for vaccines containing thimerosal as a preservative would be evaluated on a case-by-case basis.

Vaccine manufacturers say that as of 2000, vaccines recommended for use in children and infants have all been available in versions that contain no ('thimerosal-free') or only trace amounts ('thimerosal preservative-free') of the compound. The FDA has licensed new paediatric formulations of hepatitis B and diphtheria, tetanus and acellular pertussis (DTaP) vaccines. These include Merck & Co's Recombivax-HB (thimerosal free), GlaxoSmithKline's Engerix-B (trace thimerosal), and Aventis-Pasteur's Tripedia (trace thimerosal).

Thimerosal is still used in some multidose vials intended for use in the developing world. This is because where several doses are taken from the same container, thimerosal offers better protection from contamination than other preservatives such as 2-phenoxy ethanol, according to the World Health Organisation.