The claim was made most forcefully by Robert F. Kennedy Jr. in a 2005 article published in print by Rolling Stone and online by Salon, and in what Kennedy called an "original research paper" on his web site.
It's a terrific story: A cabal of scientists corrupted by pharmaceutical company cash meets secretly at a wooded enclave outside Atlanta called Simpsonwood to hide convincing evidence that autism is caused by a mercury-based vaccine preservative called thimerosal.
Kennedy, after obtaining a verbatim transcript of the conference, reveals the CDC plot. "If, as the evidence suggests, our public health authorities knowingly allowed the pharmaceutical industry to poison an entire generation of American children, their actions arguably constitute one of the biggest scandals in the annals of American medicine," Kennedy concluded.
It's such a good story, it's still frightening parents seeking information on vaccine safety -- even though by January 2011 the article had been withdrawn by both publishers and removed from their web sites.
Salon published five corrections to the story that "went far in undermining Kennedy's expose," according to Salon's editors. Critics of the Kennedy piece, including a devastating critique by Seth Mnookin in The Panic Virus, "further eroded any faith we had in the story," they wrote.
Kennedy's allegations spurred an 18-month investigation by a U.S. Senate committee. That report found allegations of CDC misconduct to be "unsubstantiated," and concluded that there was no cover-up.
So why does Kennedy's story still frighten parents? What really happened? Is mercury in vaccines still a concern? Why does Simpsonwood continue to vex people on all sides of the vaccine debate?
Early Concerns About Mercury
The FDA Modernization Act of 1997 called for the FDA to assess the risk of all foods and drugs that contained mercury. In late 1998, the FDA asked vaccine manufacturers to detail the use of mercury in vaccines.
A CDC and FDA review quickly found that in the first six months of life, children were getting a cumulative dose of mercury from vaccines that exceeded the EPA's maximum safe exposure level. All of the mercury from vaccines comes from a compound called thimerosal.
To be on the safe side, vaccine makers agreed to remove thimerosal from vaccines as soon as possible. Single-dose vaccines are now thimerosal free (some may still have tiny traces of the compound), but no replacement yet has been found for thimerosal in the multi-dose vials.
Thimerosal is a mercury compound. Ironically, it has long been used to make vaccines safer. Tiny quantities of thimerosal kept vaccines free of dangerous germs that used to cause deadly infections in healthy children.
When researchers added up all the mercury in the infant vaccines, it was 40% greater than the amount of mercury the EPA considered a safe environmental exposure. This was a shock, recalls Christopher John Clements, MD, MPH, who then was an official with the World Health Organization's immunization program.
"I was the focal point for this issue in WHO," Clements tells WebMD. "I was as distressed as the CDC to discover that there was such a small body of scientific literature published on the chemical's safety, despite its having been used in vaccines for many years."
To be safe, the CDC, FDA, the American Academy of Pediatrics, and vaccine manufacturers in 1999 agreed to remove thimerosal from all vaccines as soon as possible. This was accomplished in 2001 (except for flu vaccine, now available in thimerosal-free formulations).
But what about children already exposed -- and what about ongoing exposure while thimerosal was being removed?
Thomas Verstraeten, MD, a Dutch researcher working at the CDC, designed and led a study that looked at data from two large California HMOs. He looked for a link between neurological and developmental disorders and thimerosal exposure in 124,170 infants.
And he did indeed see a red flag. Thimerosal appeared to increase children's risk of tics, neurodevelopmental delay, attention deficit hyperactivity disorder, and language delay -- not autism, but close enough for serious concern.
The data would be presented at the July 2000 meeting of the ACIP, a panel of experts that advises the CDC and FDA on vaccine policy. To prepare the vaccination community for the news -- and to figure out how to communicate Verstraeten's preliminary findings to the public -- the CDC held a June meeting to seek the advice of outside experts: 11 "consultants" and 49 "resource specialists."
Because of other meetings being held in Atlanta at the time, the CDC had to look outside the city for a venue. A suburban conference center was open: Simpsonwood.
The Simpsonwood Conference
Kennedy's version of what happened is that the experts conspired to hide the Verstraeten data. From the meeting transcript, he selected phrases that seemed to prove his point. Then Kennedy said that the CDC bribed the Institute of Medicine to whitewash the issue (the IOM's 2001 report found no convincing evidence linking thimerosal to autism; its 2004 report reached the same conclusion).
A close reading of the publicly available transcript leaves a far different impression than Kennedy's selective excerpts suggest. You can read it here.
In the end, the Simpsonwood consultants voted that the Verstraeten study could neither confirm nor rule out a link between thimerosal and autism, and strongly called for more study.
By the time of the ACIP meeting 13 days later, Verstraeten and colleagues had largely completed a planned second phase of their study. Essentially, the same study was repeated in a different HMO.
This time, there was no warning signal. Taken together, the two studies confirmed the consensus of the Simpsonwood meeting: Because they could not establish a definite cause and effect between thimerosal and autism, further research urgently was needed to confirm or reject the link between the two.
The ACIP then unanimously voted to continue the ongoing transition to thimerosal-free vaccines, but not to explicitly recommend against the use of vaccines containing thimerosal or to postpone vaccinations until thimerosal-free versions became available.
Simpsonwood and Vaccine Safety
Reading the Simpsonwood transcript, it’s hard to see the conspiracy Kennedy describes. Paul Offit, MD, recalls being interviewed by Kennedy and infuriated at being misquoted by him.
Offit, chief of infectious diseases and director of the vaccine education center at Children's Hospital of Philadelphia, is co-inventor of the rotavirus vaccine now sold by Merck. A staunch defender of vaccination -- and a member of the ACIP in 2000 -- he's a frequent target of anti-vaccination groups.
"To believe that the Simpsonwood meeting was a conspiracy, you have to believe the people who sat around that table -- eminent toxicologists and pediatricians included -- you have to believe they were in league to hide the truth," Offit tells WebMD. "Whereas if you read that transcript, you'll see they were trying to understand the issue and where to go from there."
The focus on a Simpsonwood "conspiracy" also rankles Sallie Bernard, co-founder and president of SafeMinds, a group that advocates for research into environmental mercury as a possible cause of autism. The SafeMinds web site indicts thimerosal as a dangerous source of mercury.
But what worries Bernard about the focus on Simpsonwood is that it's a distraction from the issue of vaccine safety.
"Why the focus on Simpsonwood or on conspiracies? It's as if the concerns over vaccine safety were some narrow partisan group thinking these things up," Bernard tells WebMD.
Thimerosal and Autism
In the decade since the Simpsonwood meeting, scientists have come to a better -- but still incomplete -- understanding of what thimerosal does in the human body.
As it turned out, thimerosal is based on a form of mercury called ethyl mercury, while the EPA limits were based on methyl mercury. Why is that important? The body processes ethyl mercury far differently than it does methyl mercury.
The body takes ethyl mercury out of the blood much faster than methyl mercury, turning it into less-toxic inorganic mercury. Before the Simpsonwood conference took place, it was assumed that ethyl mercury was just as toxic as methyl mercury. Fortunately, it is not.
That's not to say that ethyl mercury has been proven 100% harmless. Ongoing studies offer reassurance, and studies of millions of children identify no risk to children with the highest levels of exposure to vaccines containing thimerosal. Nor do these studies find evidence of a subgroup of children particularly sensitive to thimerosal toxicity. But the fact that inorganic mercury accumulates in the brains of monkeys given high doses of thimerosal suggests there's more to learn about the preservative's safety.
In 2006, Clements reviewed studies of thimerosal toxicology and vaccine epidemiology. He found 12 studies published since the Simpsonwood conference. Six found no link.
Six did find a link -- all by the father-and-son research team of Mark Geier, MD, PhD, and David Geier. The Geiers’ research has been roundly criticized by medical reviewers. And it was found not credible in a 2010 decision by George L. Hastings Jr., special master of the U.S. District Court, also known as the vaccine court, which hears claims against vaccines. Hastings was charged with evaluating whether there is any evidence that thimerosal might cause autism.
Since the Clements review, studies have found no decrease in autism since thimerosal was removed from routine childhood vaccines.
After an exhaustive review of 1,284 scientific studies and the testimony of 19 experts, the U.S. vaccine court in 2010 ruled that the hypothesis linking thimerosal to autism has no basis in fact. It means that U.S. courts will no longer consider injury claims based on thimerosal in vaccines. This applies to all pending and future cases.
Although a brief Google search will turn up groups that remain convinced thimerosal somehow causes autism, the world’s scientific community has moved on.
Bernard -- who spoke at the June 2000 ACIP meeting -- feels that the research community's response to the warning signal reported by Verstraeten has been inadequate. Her point of view is that researchers have tried harder to disprove a link between thimerosal and autism than to find out what really is causing autism.
"One of the big gripes we have is there is this self-congratulation because the benefits of vaccines outweigh the risks," Bernard says. "But we could do a lot better on decreasing the risks. It is not sufficient that the benefits outweigh the risks -- you can't be so attentive to the benefit side of the equation and not to the risk side. Our point is there are many steps that can be taken to making the vaccine program safer."
Offit's gripe is with those who forget that vaccine researchers are parents, too.
"My basis is, 'Would I give this to my own children?' This is not an us-versus-them situation," he says. "To see this as a conspiracy is wrong. What do you get from that cynicism? Outbreaks of whooping cough and measles? …. There is an instinct we can appeal to -- caring for each other."