April 21--Only days after vaccine skeptics flexed their political muscle in California's Legislature, new research has shown--again-- that childhood immunizations do not contribute to a child's risk of developing autism.
In a large new study published Tuesday, researchers found that babies who were immunized against measles, mumps and rubella were no more likely to develop autism spectrum disorder (ASD) than were those who did not get the vaccine.
Even among babies who had an older sibling with the disorder--a family history that put them at higher-than-usual risk of receiving the same diagnosis--the study found no higher rate of autism in those who got the MMR vaccine than that seen in immunized babies whose elder siblings were developing normally.
All told, at least a dozen major studies have discredited the claim, first made in 1998 by the British physician Andrew Wakefield, that early childhood vaccinations--particularly those which protect against measles, mumps and rubella--are responsible for autism's development in children.
In 2010, Wakefield was found to have falsified his research, and was barred from practicing medicine in the United Kingdom. But his claims of a link between the MMR vaccine and autism continue to resonate with a minority of American parents, despite consistent findings that a child's vaccination does not affect his likelihood of developing autism, the age at which he might develop the disorder, or the severity of his autism, should he develop it.
Dr. James Cherry, a pediatric infectious disease specialist at UCLA who was not involved with the current study, said it's "not surprising" that the latest study fails to find a signal indicting the MMR vaccine as the cause of autism.
Despite the study's high quality, Cherry added, "8 million studies are not going to convince people who believe" there is a cause-and-effect relationship between the MMR vaccine and autism to believe otherwise.
In the wake of a three-month measles outbreak linked to unvaccinated children exposed to the measles virus at Disneyland in Anaheim, lawmakers in California and elsewhere have sought to close loopholes that give parents wide latitude to refuse vaccinations for their children. But last week, protests by hundreds of parents derailed--at least temporarily-- a measure making its way through the California Senate's Health Committee that would have required vaccination of virtually all children as a condition for attending public and private schools.
In California, public health officials have said that low vaccination rates, evident throughout the state but glaring in five geographical clusters, are almost certain to spur new outbreaks of vaccine-preventable childhood diseases.
Published in JAMA, the latest research mined the records of 95,727 children born into families covered by a large commercial health plan to detect any link between vaccination and a diagnosis of autism spectrum disorder. All of the babies had at least one older sibling, and 1,929 of the children had a sibling who had already been diagnosed with ASD.
Epidemiological studies have established that, compared to babies whose elder siblings are developing normally, such children are at higher risk themselves of developing an autism spectrum disorder. Even if there were only a weak association between the MMR vaccine and autism, the authors reasoned, it would show up clearly in this "risk-enhanced population."
It did not. Indeed, on a first look at the statistics, the study's findings would suggest that, among all younger siblings, getting the MMR vaccine might have conferred some protection against development of ASD. The authors of the study--led by Dr. Anjali Jain of the Virginia-based Lewin Group--discounted that likelihood. The lower autism rates among those who got the vaccine were more likely a function of parents' decisions to defer vaccination when a child showed early social or communications delays, the authors wrote.
By hunting for--and failing to find--a link between childhood vaccination and autism in children known to be at higher risk for the disease, the latest research further discredits the purported link between autism and vaccines.
In an editorial published Tuesday alongside the JAMA study, psychiatrist Bryan H. King of Seattle Childrens Hospital acknowledged that while "abundant" evidence has failed to find a link between the MMR vaccine and autism in the general population, it remained legitimate to look for such a link in a smaller, more vulnerable population.
"Could it be that if all the requisite genetic and other risks are present, MMR can lead to the development of autism?" he asked. "If so, the population in which there might be such a signal would be families already affected by autism," he added.
By showing such fears to be unfounded, wrote King, the current study should help allay concerns that have prompted some parents of a child with autism to delay or forego vaccinating subsequent children.
A recent study showed that, in a sample of 486 parents of a child with autism, nearly 20% said they had refused or delayed having a subsequent child immunized.
A similar pattern was clear in the current study: by the time they were 2, 84% of children with an older sibling unaffected by ASD had gotten at least one shot of MMR vaccine, while 73% of 2-year-olds with an affected sibling had done so. By 5, 92% of kids with an unaffected older sibling had been fully vaccinated against measles, mumps and rubella, while 86% of those with an affected sibling had gotten both doses.
But the current study's failure to find an MMR-autism link in kids at higher risk of the disorder does more than offer reassurance, wrote Dr. King. It helps "move the field forward towards a more focused and productive search for more temporal and environmental factors that contribute to autism risk," he wrote.
Johns Hopkins University autism researcher M. Daniele Fallin agrees that, given the mounting evidence, "we should move on" to exploring more promising explanations for autism.
Over the last decade, added Fallin, who was not involved in the current study, the search for autism's causes have shifted definitively away from the infant and toddler years and focused increasingly on factors at play "mostly in utero and even before."
Fallin, who chairs the Bloomberg School of Public Health's department of mental health, is co-author of recent research that looked at DNA from the sperm of men whose children had early signs of autism, and found distinct patterns in the "epigenetic tags" that help regulate the activity of genes.
In that small study, published last week in the International Journal of Epidemiology, the epigenetic sites most consistently seen in fathers of children with autism were close to genes that played a role in developmental processes, particularly brain development.
Other findings, published last week in JAMA, have found that babies born to mothers who had type-2 diabetes before becoming pregnant had a roughly 20% higher likelihood of developing autism spectrum disorder than those who did not. Babies of mothers who developed gestational diabetes before 26 weeks' gestation--but not late in pregnancy-- were at even higher risk of developing ASD, suggesting that metabolic abnormalities to which fetuses are exposed during a crucial period of brain development early in pregnancy may play some role in the disorder.
