{UAH} Vaccines and Autism: A Tale of Shifting Hypotheses

Although child vaccination rates remain high, some parental concern persists that vaccines might cause autism. Three specific hypotheses have been proposed: (1) the combination measles-mumps-rubella vaccine causes autism by damaging the intestinal lining, which allows the entrance of encephalopathic proteins; (2) thimerosal, an ethylmercury-containing preservative in some vaccines, is toxic to the central nervous system; and (3) the simultaneous administration of multiple vaccines overwhelms or weakens the immune system. We will discuss the genesis of each of these theories and review the relevant epidemiological evidence.

A worldwide increase in the rate of autism diagnoses—likely driven by broadened diagnostic criteria and increased awareness—has fueled concerns that an environmental exposure like vaccines might cause autism. Theories for this putative association have centered on the measles-mumps-rubella (MMR) vaccine, thimerosal, and the large number of vaccines currently administered. However, both epidemiological and biological studies fail to support these claims.

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MMR

On 28 February 1998, Andrew Wakefield, a British gastroenterologist, and colleagues [1] published a paper in The Lancet that described 8 children whose first symptoms of autism appeared within 1 month after receiving an MMR vaccine. All 8 of these children had gastrointestinal symptoms and signs and lymphoid nodular hyperplasia revealed on endoscopy. From these observations, Wakefield postulated that MMR vaccine caused intestinal inflammation that led to translocation of usually nonpermeable peptides to the bloodstream and, subsequently, to the brain, where they affected development.

Several issues undermine the interpretation by Wakefield et al. [1] of this case series. First, the self-referred cohort did not include control subjects, which precluded the authors from determining whether the occurrence of autism following receipt of MMR vaccine was causal or coincidental. Because ~50,000 British children per month received MMR vaccine between ages 1 and 2 years—at a time when autism typically presents—coincidental associations were inevitable. Indeed, given the prevalence of autism in England in 1998 of 1 in 2000 children [2], ~25 children per month would receive a diagnosis of the disorder soon after receiving MMR vaccine by chance alone. Second, endoscopic or neuropsychological assessments were not blind, and data were not collected systematically or completely. Third, gastrointestinal symptoms did not predate autism in several children, which is inconsistent with the notion that intestinal inflammation facilitated bloodstream invasion of encephalopathic peptides. Fourth, measles, mumps, or rubella vaccine viruses have not been found to cause chronic intestinal inflammation or loss of intestinal barrier function. Indeed, a recent study by Hornig et al. [3] found that the measles vaccine virus genome was not detected more commonly in children with or without autism. Fifth, putative encephalopathic peptides traveling from the intestine to the brain have never been identified. In contrast, the genes that have been associated with autism spectrum disorder to date have been found to code for endogenous proteins that influence neuronal synapse function, neuronal cell adhesion, neuronal activity regulation, or endosomal trafficking [4].

Although no data supporting an association between MMR vaccine and autism existed and a plausible biological mechanism was lacking, several epidemiologic studies were performed to address parental fears created by the publication by Wakefield et al. [1] (table 1). Fortunately, several features of large-scale vaccination programs allowed for excellent descriptive and observational studies—specifically, large numbers of subjects, which generated substantial statistical power; high-quality vaccination records, which provided reliable historical data; multinational use of similar vaccine constituents and schedules; electronic medical records, which facilitated accurate analysis of outcome data; and the relatively recent introduction of MMR vaccine in some countries, which allowed for before and after comparisons.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2908388/?fbclid=IwAR0YF14gcThrDT35bgOKVbhiZfcgnNxSj1j0Dm0mTbWPvHzXMFQkvHV2iLc  

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Allaah gives the best to those who leave the choice to Him."And if Allah touches you with harm, none can remove it but He, and if He touches you with good, then He is Able to do all things." (6:17)

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