For years, the scientific evidence has been accumulating. The latest, published this week, once again showed that thimerosal, a mercury-based preservative long used in childhood vaccines, does not cause the neurological disorders associated with the U.S. autism epidemic.
In fact, scientists at the California Department of Public Health demonstrated that in the years since nearly all thimerosal was removed from vaccines in 2001, the rate of autism has continued to rise there. Had thimerosal been the culprit, those numbers should have decreased.
Not in the world of autism, an emotionally charged place where Web sites, blogs and parent advocacy groups have spent a decade promoting the theory that the thimerosal injected into babies beginning when they were just weeks old has left as many as one in 150 children disabled by autism. These advocates have lobbied Congress, screamed about coverups and filed financial claims against the government.
"I know the people who want to believe it's thimerosal will find fault with this [study]," said Dr. Gary Goldstein, president & CEO of the Kennedy Krieger Institute in Baltimore. "The scientists, they're saying, 'Why are we still talking about this?' "
Thimerosal, which is almost 50 percent mercury, has been a preservative in vaccines since the 1930. The first case of autism was identified at Johns Hopkins University in the 1940s.
Before 1991, only one vaccine - the one for diphtheria, tetanus and pertussis - contained thimerosal. Starting in 1991, two more were added to the schedule for infants - sharply increasing the quantity of mercury given to young children. Most other vaccines have remained mercury-free.
By 1999, some government scientists were concerned that infants might be getting too much mercury. As a precautionary measure, the federal Centers for Disease Control and Prevention and the American Academy of Pediatrics asked pharmaceutical companies to remove thimerosal from vaccines.
Still, according to Dr. Paul A. Offit, the infectious diseases chief at Children's Hospital of Philadelphia, doctors kept insisting that parents not worry about the safety of vaccines.
But the firestorm came. "Many parents, frightened by a sudden change in policy, reasoned that thimerosal was targeted because it was harmful - and their faith in the vaccine infrastructure was shaken," Offit wrote in a September issue of The New England Journal of Medicine.
By 2002, the last U.S. doses of required vaccines for children made with thimerosol had expired, though other countries continue to use it.
Those who publicly deny the link between thimerosal and autism have been harassed - some have received death threats. Goldstein said he has gotten hate e--mails and a call saying, "I hope your child dies."
In the new California study, published in Archives of General Psychiatry, researchers looked at data reported to the California Department of Developmental Services, which serves the state's disabled population. "To me, the body of evidence is persuasive," said study author Dr. Robert Schechter.
But Lyn Redwood, a co-founder of SafeMinds, a nonprofit that raises awareness of mercury exposure in children, said she isn't ready to write off the thimerosal/autism connection. That's because thimerosal has not disappeared. Pregnant women receive flu shots - which contain a full dose of thimerosal - and flu vaccines for babies often contain it, she said.
"Our children are still getting exposed to mercury," said the Atlanta nurse practitioner and mother. "I think mercury should still be on everybody's radar screens."
She said the California data are flawed because the continued rise in autism could be explained by immigrants there who were vaccinated in other countries where thimerosal is still used.
"I think it's a little bit early to close the books on thimerosal," she said.
Dr. Mark Geier, a Silver Spring epidemiologist, agreed. "Everyone worries about lead poisoning," he said. "Do you know how much more dangerous mercury is?"
It is still unknown what causes autism, a spectrum of disorders marked by impairment in social development and communication. Autistic children usually exhibit language delays and often have difficulties relating to people, failing to read basic social cues in the faces of others.
Most specialists believe autism's cause will be found in genetics, but many also suspect that an environmental trigger contributed to the explosive growth in autism cases in the past generation. Doctors also attribute some of the growth to better diagnosis and a broadening of autism's definition.
But it's the scientific community's inability to provide a definitive cause that has led so many to thimerosal. "The final answer to me will come when that paper is written that says, 'This is what causes autism'," said Dr. Richard Deth, a pharmacologist at Northeastern University in Boston, who calls thimerosal "a prime suspect."
Some health experts are concerned that thimerosal worries might keep some parents from vaccinating their children, forgetting the value of protecting them from diseases that were once major killers.
"Immunizations have kind of become the fall guy for problems," said Dr. Andrew W. Zimmerman, a pediatric neurologist at Kennedy Krieger, "but the public has a short memory - because before immunizations, the diseases themselves were much worse."