The study found that the additive - known as thiomersal in the US - could cause behavioural abnormalities in newborn mice that have a specific genetic susceptibility, adding fuel to the fire of those who still argue that thiomersal is linked to the development of autism.
Because there has been work suggesting that susceptibility to autism is linked with autoimmune conditions, the researchers used a mouse model called SJL/J that is prone to develop autoimmunity to explore the effects of thiomersal exposure.
They administered thiomersal to the mice at doses that would mimic exposure in human children. The mice showed growth delay; reduced locomotion; exaggerated response to novelty; and changes in the morphology of neurons in the hippocampus of the brain, with altered glutamate receptors and transporters. Strains resistant to autoimmunity showed no such changes.
The hypothesis that thiomersal was responsible for developmental delays in children first surfaced in 1999, and since then has been extended to autism. But the link has been discredited by bodies such as the Institute of Medicine in the US, which just last month completed a review of the clinical and epidemiological data on thiomersal and concluded that there was no link with autism.
Moreover, almost all the paediatric vaccines on the market have been modified as a precautionary measure to use an alternative preservative such as 2-phenoxy ethanol (2-PE) said a spokesperson for GlaxoSmithKline Biologicals, a vaccine maker based in Belgium.
She noted that thiomersal has been used in two different ways in vaccine production. It is used in the production process to maintain sterility, but processing means that only trace amounts are present at the end. Before the shift away from its use as a preservative, it would also have been added at the end of the process to function as a preservative in the vial.
As of 2000, vaccines recommended for use in children and infants have all been available in versions that contain no ('thiomersal-free') or only trace amounts ('thiomersal preservative-free') of the compound, although there is one exception.
Some multidose vials intended for use in the developing world still use thiomersal, but there are sound reasons for this according to the World Health Organisation. For vaccines used in multidose formulations thiomersal offers better protection from contamination than other preservatives such as 2-phenoxy ethanol, it maintains.
Meanwhile, the US National Network for Immunisation Information has issued a statement saying that it is important to keep the new study in perspective.
"Abnormalities were noted only in mice that were specially bred to have problems with their immune systems. How and whether this relates to human infants can only be determined by additional research," it said in a statement.
In the US, thiomersal is no longer used as a preservative in most childhood vaccines, but a possible exception is the influenza vaccine - although a thiomersal-free version is available.