As for thimerosal, government precautions notwithstanding, it was never clear how threatening it might be. The federal mercury standards that first heightened concern were developed for methylmercury, not ethylmercury, the form contained in thimerosal. Ethylmercury has less risk of accumulating to a toxic dose because it does not last as long in the body. And, according to the IOM’s 2004 report, there had never been any evidence of a major incident of mercury poisoning leading to autism.
The strongest argument against the idea that thimerosal poisoned a generation of children does not emerge from the body of published studies alone. There is the added detail that although thimerosal is no longer present in any recommended childhood vaccines save the inactivated influenza vaccine—and hasn’t been, beyond trace amounts, since 2001—no one is hailing the end of autism. “If you thought thimerosal was related to autism, then the incidence of autism should have gone down,” Harvard’s McCormick explains. “And it hasn’t.”
In 2005 David Kirby stated that if autism rates didn’t begin to decline by 2007, “that would deal a severe blow to the autism-thimerosal hypothesis.” But as McCormick notes, despite the absence of thimerosal in vaccines, reports of autism cases have not fallen. In a 2008 study published in Archives of General Psychiatry, two researchers studying a California Department of Developmental Services database found that the prevalence of autism had actually continued increasing among the young. Kirby concedes that these findings about the California database represent a “pretty serious blow to the thimerosal-causes-autism hypothesis,” though he does not think they thoroughly bury it. In an interview, he outlined many problems with relying on the California database, suggesting potential confounding factors such as the state’s high level of immigration. “Look, I understand the desire to try to end this and not scare parents away from vaccination,” Kirby says. “But I also feel that sometimes that desire to prove or disprove blinds people on both sides.”
Kirby says—and even some vaccine defenders agree—that some small subgroup of children might have a particular vulnerability to vaccines and yet be missed by epidemiological studies. But the two sides disagree as to the possible size of that group. “If one or two or three children every year are getting autism from vaccines, you would never pick that up,” Offit says. Kirby, in contrast, feels that while the idea of thimerosal as the “one and only cause of autism has gone out the window,” he still believes there is an “epidemic” with many environmental triggers and with thimerosal as a possible contributing factor.
Meanwhile, in the face of powerful evidence against two of its strongest initial hypotheses—concerning MMR and thimerosal—the vaccine skeptic movement is morphing before our eyes. Advocates have begun moving the goalposts, now claiming, for instance, that the childhood vaccination schedule hits kids with too many vaccines at once, overwhelming their immune systems. Jenny McCarthy wants to “green our vaccines,” pointing to many other alleged toxins that they contain. “I think it’s definitely a response to the science, which has consistently shown no correlation,” says David Gorski, a cancer surgeon funded by the National Institutes of Health who in his spare time blogs at Respectful Insolence, a top medical blog known for its provaccine stance. A hardening of antivaccine attitudes, mixed with the despair experienced by families living under the strain of autism, has heightened the debate—sometimes leading to blowback against scientific researchers.
Paul Shattuck did not set out to enrage vaccine skeptics and the parents of autistic children. Currently an assistant professor at the George Warren Brown School of Social Work at Washington University in St. Louis, he has dedicated the last decade of his professional life to helping people with autism in their families. “Some of my dearest friends have kids with autism,” he says.
But in 2006 Shattuck came under fire after he published an article in the journal Pediatrics questioning the existence of an autism epidemic. No one doubts that since the early 1990s the number of children diagnosed with autism has dramatically increased, a trend reflected in U.S. special education programs, where children enrolled as autistic grew from 22,445 in 1994–1995 to 140,254 in 2003–2004. Yet Shattuck’s study found reasons to doubt that these numbers were proof of an epidemic. Instead, he suggested that “diagnostic substitution”—in which children who previously would have been classified as mentally retarded or learning disabled were now being classified on the autism spectrum—played a significant role in the apparent increase.
Shattuck did not reject the idea that rising autism levels might be in part due to environmental causes; he merely showed the increase was largely an artifact of changing diagnostic practices, which themselves had been enabled by rising levels of attention to autism and its listing as a diagnostic category in special education. Yet simply by questioning autism epidemic claims in a prominent journal, he became a target. “People were obviously Googling me and tracking me down,” he recalls. Shattuck emphasizes that most e-mails and calls merely delivered “heartfelt pleas from people with very sick kids who’ve been led to believe a particular theory of etiology.” The bulk weren’t menacing, but a few certainly were.
Others attacked Shattuck’s research on the Web and insinuated that he had fabricated his data or committed scientific misconduct. “It was dismaying to feel like people were calling me a traitor to autistic kids and families,” he says.
“If there has been a more harmful urban legend circulating in our society than the vaccine-autism link,” University of Pennsylvania bioethicist Arthur Caplan wrote in The Philadelphia Inquirer, “it’s hard to know what it might be.” One type of harm, as Shattuck’s story shows, is to individual scientists and the scientific process. There is a real risk that necessary research is being held back as scientists fear working in such a contested field. Shattuck’s experience is not unique. Offit cannot go on a book tour to promote Autism’s False Prophets because of the risk involved in making public appearances. He has received too many threats.
Yet another cost comes in the rush toward unproven, and potentially dangerous, alternative therapies to treat autism. It is easy to sympathize with parents of autistic children who desperately want to find a cure, but this has led to various pseudoremedies whose efficacy and safety have been challenged by science. These include facilitated communication, secretin infusion, chelation therapy (which involves pumping chemicals into the blood to bind with heavy metals such as mercury), and hormonal suppression. It is estimated that more than half of all children with autism are now using “complementary and alternative” treatments.
Disease, however, is the greatest danger associated with holding back vaccines amid the ongoing investigation of dubious claims. Both the vaccinated and the unvaccinated populations are placed at greater risk. Given enough vaccine exemptions and localized outbreaks, it is possible that largely vanquished diseases could become endemic again. (That is precisely what happened with measles in 2008 in the U.K., following the retreat from the MMR vaccine in the wake of the 1998 scare.) The public-health costs of such a development would be enormous—and they would not impact everyone equally. “If vaccine rates start to drop, who’s going to get affected?” Peter Hotez asks. “It’s going to be people who live in poor, crowded conditions. So it’s going to affect the poorest people in our country.”
Paradoxically, the great success of vaccines is a crucial reason why antivaccination sentiment has thrived, some scientists say. Most of the diseases that vaccines protect against have largely been licked. As a consequence, few people personally remember the devastation they can cause. So with less apparently on the line, it is easier to indulge in the seeming luxury of vaccine skepticism and avoidance. Even before the recent spike in attention to thimerosal, members of the public were alarmingly skeptical of vaccines. In a 1999 survey, 25 percent felt their children’s immune systems could be harmed by too many vaccinations, and 23 percent shared the sentiment that children receive more vaccinations than are healthy. There is every reason to think that those numbers—gathered before the vaccine-autism controversy reached anything like its current intensity—have risen since.