A new study may be the latest nail in the coffin of a theory that draws a link between the mercury-containing vaccine additive thimerosal and autism.
The research is the latest to contradict concerns over childhood vaccinations as a possible cause of autism -- concerns that have gained publicity in the past decade as the number of children diagnosed with the disorder climbs steadily in the United States.
Because vaccinations are mandatory for all children at a certain age, some parents and doctors believe that the mercury once found in many childhood vaccines may contribute to the development of autism. However, so far a number of large scientific studies have shown no association between thimerosal and autism.
And the most recent research to nullify this association, published Monday in the journal Archives of General Psychiatry, reveals that the prevalence of autism for children ages 3 to 12 continued to increase in California even after 2001 -- when all but trace levels of mercury had been removed from most childhood vaccines.
"If thimerosal exposure is a primary cause of autism, then the prevalence of autism would be predicted to decrease, as young children's exposure to thimerosal has sharply decreased to its lowest levels in decades," noted lead study investigator Robert Schechter in the commentary section of the research.
Before 2001, thimerosal was used in many childhood vaccines to prevent microbial contamination. However, in July 1999, the U.S. Public Health Service implemented a precautionary measure removing thimerosal -- which contains 49.6 percent ethylmercury -- from all childhood vaccines.
To determine whether reduced exposure to thimerosal led to a decrease in autism cases, researchers at the California Department of Public Health analyzed data from the California Department of Developmental Services on the prevalence by age and birth cohort of children with autism between 1995 and 2007.
But instead of finding a decrease in autism following the elimination of thimerosal from most vaccines, researchers found that for each quarter from 1995 to the end of 2003, the prevalence of autism in children between the ages of 3 and 5 years increased from 0.6 to 2.9 per 1,000 births.
From 2004 to 2007, when exposure to thimerosal from childhood vaccinations vastly declined, the prevalence of autism in children between the ages of 3 and 5 years increased from 3.0 to 4.1 per 1,000 births.
The findings bolster the position that has been steadfastly held by the Institute of Medicine, and many doctors worldwide, that no link can be established between thimerosal and autism.
Some researchers say that the absence of a link has already been demonstrated in Denmark, where thimerosal was removed from all vaccines in 1992. Despite this move, autism rates in that country continued to rise until 2004.
"Conspiracy theorists will continue to engage this theory on the link between autism and vaccinations, but hopefully, they will take into account that this idea has been refuted on an international scale, not just by the CDC," said Mark Slifka, associate professor in the Vaccine and Gene Therapy Institute at Oregon Health Sciences University.
"Studies out of Sweden and Denmark, independent studies on an international basis, have all come to this same conclusion."
Although the latest study to deny the link between autism and thimerosal does not seek to explain possible causes for the overall increase in diagnosed autism cases across the country, many doctors believe the increase is due simply to heightened awareness of the disorder and improved methods of diagnosing children with autistic behaviors.
"Physicians know a lot more about autism and are much more aware of the wide spectrum of autistic disorders," said Dr. Robert Davis, director of the Center for Health Research Southeast at Kaiser Permanente and former director of the CDC's Immunization Safety Office. "Doctors today are much more likely to classify child or adolescents with minor characteristics of autism as being autistic, even though 20 years ago we might have labeled them differently -- as having a speaking disorder or even a retardation."
While many doctors continue to hope this study provides the final nail in the coffin for the link between autism and thimerosal, Dr. Peter Hotez, professor and chairman of the department of microbiology, immunology and tropical medicine at George Washington University, believes the debate over this issue has already done damage to progress against the disorder.
"Unfortunately, this whole dialogue around vaccines and autism has been a distraction," Hotez says. "It's focused around an erroneous association between vaccines and autism, and it's sucked away resources from what we really need, which is more and better research into causes of autism and support for families."