THURSDAY, May 20, 2004 (HealthDayNews) -- In a large-scale report aimed at snuffing out any embers of doubt, a U.S. government panel has concluded decisively that childhood vaccinations do not cause autism.
Since concerns about a suspected link were first raised in the late 1990s, a barrage of articles have refuted it. The latest Institute of Medicine (IOM) report collected all the available evidence, essentially making a cannonball out of scattered pistol fire.
The report referred specifically to the measles-mumps-rubella (MMR) vaccine as well as to vaccines that at one point contained the mercury-based preservative thimerosal. Considerable debate has brewed for years as to whether these might trigger the onset of autism in small children.
Although it's not clear whether this debate will ever be put completely to rest, the report is "awful darn close to the final word," said Dr. Tom Saari, a professor of pediatrics at the University of Wisconsin Medical School in Madison and a member of the American Academy of Pediatrics (AAP) committee on infectious diseases.
Researchers' time would be better spent pursuing other culprits, experts added.
"It's clearly not going to settle it for some people, but it clearly suggests that there are other avenues that are much more profitable to pursue in terms of trying to understand what autism is and how to both prevent and treat it," said Dr. Marie C. McCormick, head of the IOM Immunization Safety Review Committee and a professor of maternal and child health at the Harvard School of Public Health.
The controversy regarding autism and vaccines began in 1998 with the publication of a case series describing 12 children with pervasive developmental disorder, eight of whom exhibited behavioral problems their parents and physicians said coincided with receiving the MMR vaccine.
This question of timing is one argument supporters of the vaccine-autism link use to bolster their point of view. "The timing issue certainly affects the perception of an association between MMR and autism, because MMR is given in the second year of life, which is when symptoms of autism tend to be more evident," McCormick said.
Another issue is the fact that, until 1999, thimerosal was used in more than 30 vaccines licensed and marketed in the United States. That year, the U.S. Food and Drug Administration determined that small children receiving the standard vaccination schedule might be exposed to cumulative doses of ethylmercury that exceeded some safety guidelines. Measures were quickly taken to remove thimerosal from these vaccines.
The IOM had already published two reports on possible links between autism, the MMR vaccine and thimerosal. Those reports concluded there was no evidence to support a link between MMR and autism, but there was not enough evidence to disprove a thimerosal-autism connection. In any event, today vaccines given to infants either have no thimerosal or negligible amounts.
Thimerosal is still included in some influenza vaccines but, Saari said, "The potential exposure a child would get from a flu vaccine is extremely small." And there are thimerosal-free versions available this year, albeit not enough to inoculate everybody.
For the current study, the committee looked at existing research on the subject, namely five studies that explored the thimerosal-autism connection and 14 that looked at MMR and autism. The five thimerosal studies found no association between that compound and autism, while the 14 MMR studies also found no credible connection.
"We have 12 well-designed studies that say it isn't so," Saari said. "In that particular arena, I think most of us feel very comfortable with the weight of the evidence."
In addition, the committee determined that five studies found a link between thimerosal and autism, and another two found a connection between MMR and the disease. The panel concluded the evidence presented was not compelling. "They were not high enough quality to sway the conclusions of the other reports," McCormick stated.
Finally, the committee also concluded that there was insufficient evidence that activation of the immune system by the vaccine somehow triggered autism.
But not all researchers are willing to end their explorations.
Dr. Timothy Buie, an instructor of medicine at Harvard Medical School, is in the middle of a study to see if the measles virus is present in the colon of kids with autism. "We are trying to find out if the presence of the virus is any more relevant in autistic kids. We're not sure what we're going to find," he explained.
The original 1998 case studies saw GI symptoms in the children that seemed to point to a link between autism and the MMR vaccine.
"I'm not certain there is enough information out there to draw that conclusion [of the IOM]," Buie said. "There are still not a lot of centers actively looking at the question. I think they do deserve a little bit more time and a few more centers to put their two cents in to see if they can find for or find against. Anything that would lead us to more concern or less concern is of value."