Dec 12, 2007
The prevalence of autism has risen dramatically in the past two decades. Prevalence is an estimate of the number of affected persons at a point in time. Prevalence studies conducted in the United States show that the prevalence of ASDs in 2003-2004 is somewhere between 1 in 139 and 1 in 181 children between the ages of 4 and 17. This implies that there are around 300,000 children diagnosed with an ASD. Most scientists feel the rise in prevalence is due to a combination of changes in the diagnostic criteria for autism spectrum disorders (ASDs) and increased awareness of the disorder (e.g., Wing and Potter, 2002).
However, some have suggested that the increased prevalence of ASDs corresponds with an increase in the number of vaccinations recommended for children. Correlation of two events is not sufficient evidence to assert that one caused the other as the two events could be unrelated. For example, if a child is born during a full moon one could suggest that the lunar event caused the birth. This would be an example of a likely false correlation. Therefore, further study of such correlations is necessary to reveal evidence to either support or disconfirm a causal hypothesis. One specific hypothesis of vaccines being linked to autism suggests that thimerosal, a preservative previously used in childhood vaccines that was removed from vaccines manufactured in the US in 1999, can cause autism. Thimerosal is still present in some versions of the flu vaccine. Several versions of this theory target different mechanisms for how thimerosal damages the child and causes autism. They all, however, state that some damage occurs to the developing child after vaccination. Advocates of the “thimerosal causes ASDs” hypothesis have also suggested that the prevalence of ASDs will substantially decrease subsequent to thimerosal being removed from childhood vaccines.
A recent study published in Pediatrics by Fombonne and colleagues (2006) has shown that decreased thimerosal in childhood vaccines is not correlated with decreased prevalence of ASDs. Fombonne and colleagues have collected data on the prevalence of ASDs in children in Montreal from 1987 through 1998. Thimerosal was removed from vaccines in Canada by 1996. Fombonne et al. found that there was a statistically significant increase in the prevalence of ASDs for children who received thimerosal-free versions of the recommended childhood vaccines. If one were to apply the false correlation influenced thinking here it could be suggested that thimerosal should be added back into vaccines because they are correlated with a lower prevalence of autism. Similar findings to Fombonne et al. have been obtained in Denmark and Sweden and will likely be revealed in the US in the not too distant future. Informal review of several educational data systems suggests that the prevalence of ASDs is continuing to increase in the US.
Another hypothesis suggests that the measles-mumps-rubella (MMR) vaccine causes autism. ASDs clearly have a genetic origin but one environmental trigger has been identified. If a pregnant mother contracts rubella, there is an increased chance of the child having an ASD (Chess et al., 1971/74/77). As the MMR vaccine decreases the chance of rubella infections, its use should decrease the likelihood of this environmental triggering of autism. However, some have posited that the MMR vaccine triggers regression that is sometimes part of the course autism (i.e., some children with ASDs lose skills that they previously acquired in early development). Most of the popularization of this theory has been spurred by Andrew Wakefield. He suggested that the MMR injures a child’s gut in a specific manner producing problems similar to those caused by irritable bowel syndrome. He also stated that measles was the cause because he and his colleagues purportedly detected components of the measles virus in the gastrointestinal tracts and blood of children with autism that were not present in typically developing children.
Another study recently published in Pediatrics attempted to determine whether measles was more likely to be found in the bodies of children with ASDs than in typically developing children. D’Souza and colleagues (2006) collected the largest sample of subjects for this type of study and used the same technique, polymerase chain reaction assays, that had purportedly detected measles in children with ASDs. They found that this technique produced many positive reactions in both children with ASDs and typical children. However, these reactions were further analyzed and found to be false positives for all subjects. The products of the reactions were cloned and genetically sequenced and none of these sequences contained the components of the measles virus. That is, neither the children with ASDs nor the typical children showed any evidence of measles virus in their bodies. Furthermore, there were no differences found in anti-measles antibodies across the study groups of children.
This, taken in combination with numerous other studies showing no relation between the MMR vaccine and ASDs, provides fairly definitive evidence against the “MMR causes autism” hypothesis. It should also be noted that Andrew Wakefield was found to have been paid over $150,000 by a group seeking to pursue litigation against vaccine manufacturers in the United Kingdom. Once this severe conflict of interest was revealed, ten of Wakefield’s co-authors requested that their names be withdrawn from the original publication used as support for this hypothesis. More information on Andrew Wakefield can be found at BrianDeer.com, a website maintained by the investigative journalist who revealed the conflict of interest
D’Souza et al., (2006). No Evidence of Persisting Measles Virus in Peripheral Blood Mononuclear Cells From Children With Autism Spectrum Disorder. Pediatrics, 118(4), 1744-1745.
Fombonne et al., (2006). Pervasive Developmental Disorders in Montreal: Prevalence and Links with Immunizations. Pediatrics, 118(1), 139-150. 37.
Wing, L. & Potter, D. (2002). The epidemiology of autistic spectrum disorders: Is the prevalence rising? Mental Retardation and Developmental Disabilities Research Reviews, 8(3), 151-161