Scientists have raised fears that a mercury-based preservative used in vaccines may cause symptoms of autism.
Vaccinations contain preservatives
US researchers at Columbia University found autism-like damage in the brains of mice exposed to thimerosal.
In the UK it is used in the DTP jab for diphtheria, tetanus and whooping cough and some flu jabs.
The study, in Molecular Psychiatry, has been challenged by various expert groups, who say there is no evidence that the preservative poses any risk.
A major review carried out by the US Institute of Medicine published last month found no evidence that thimerosal was linked to autism.
Similarly, investigations by the UK Committee on Safety of Medicine, Europe's Agency for the Evaluation of Medicinal Products and the World Health Organization concluded the preservative was safe.
However, the Columbia team said they found that mice exposed to thimerosal showed signs of changed behaviour, and brain abnormalities. The animals had been bred to be vulnerable to developing disorders of the immune system.
They argued it was possible that children with similarly compromised immunity may also be at risk.
Lead researcher Dr Mady Hornig said the Institute of Medicine may have jumped to premature conclusions.
She said: "We feel very strongly that any future immunisation programme is well built
on careful analysis that doesn't stall areas of research before they have been
A Department of Health spokesman said: "Vaccine safety is of paramount importance and all vaccines used in the UK are tested for their safety and efficacy."