May 16, 2007 -- New research adds to the evidence showing no causal link between autism and the mercury-containing preservative thimerosal.
Researchers compared records of children exposed to thimerosal while in the womb and those not exposed and found no difference in autism rates.
The study is one of the first to look at exposure to mercury-containing vaccines during the last months of pregnancy.
Rh Protein and Thimerosal
The Rh protein shot is routinely given to pregnant women who are Rh-negative. These women lack the Rh protein on the surface of their red blood cells. The injection prevents Rh disease, which can cause the fetus to be stillborn or lead to fatal illnesses in the newborn.
It has been suggested that children born to mothers who are given injections of Rh protein during pregnancy may be at increased risk for autism because thimerosal was used in the Rh protein injection until 2001.
"We hypothesized that if thimerosal were associated with the development of autism, we would find a higher proportion of Rh negative mothers of children with autism born before 2002," University of Missouri researchers Judith H. Miles, MD, PhD, and T. Nicole Takahashi write in the May issue of the American Journal of Medical Genetics.
'Time to Move On'
To test the theory, the two researchers determined the Rh status of 214 mothers who had children with an autism spectrum disorder and reviewed medical records to determine if the Rh-negative women had received the Rh protein vaccine late in pregnancy.
Rh-negative status was found to be no higher in the mothers of the autistic children than in the general population, nor was there more exposure to thimerosal-containing Rh vaccine in autistic children prior to birth.
Though she knows the findings won't change the minds of those convinced that thimerosal exposure causes autism, Miles tells WebMD that there is still no credible scientific proof of a link.
In 2004, an expert panel convened by the Institute of Medicine (IOM) to review the data on vaccines and autism came to the same conclusion.
"It is time to move on and focus our research dollars and efforts on avenues that will be more productive," Miles says.
Thimerosal Still in Vaccines
While the Rh vaccine and most vaccines given to children in the U.S. no longer contain thimerosal, the inexpensive preservative is still widely used to keep vaccines affordable in other parts of the world.
The researchers say that their study and others like it should reassure those who still question the safety of vaccines containing thimerosal.
"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," they write.