Back in the late 1990s, the Centers for Disease Control, vaccine manufacturers and other groups agreed that thimerosal should be removed from vaccines. As a result, childhood vaccines in the United States cease to include thimerosal in their formulation. Before that decision, thimerosal was used as a preservative in vaccine to prevent the growth of bacteria or fungi in multi-dose vials of vaccines. Without thimerosal, multi-dose vials ran the risk of being contaminated when opened, allowing bacteria and fungi to grow and posing a threat of infection or allergic reactions. (We should note that thimerosal is not used in live-virus vaccines, as it would kill the viruses in those vaccines.)
The reason for the removal of thimerosal from vaccines was that anti-vaccine groups and individuals began to spread the fear of thimerosal as a causative agent of autism. This was because thimerosal is an organic (carbon-containing) compound that contains mercury as a salt. From there, the fear of mercury took hold. For example, :
“Mercury poisoning and autism both affect the central nervous system but the specific sites of involvement in brain and the brain cell types affected are different in the two disorders as evidenced clinically and by neuropathology. Mercury also injures the peripheral nervous system and other organs that are not affected in autism. Nonspecific symptoms such as anxiety, depression, and irrational fears may occur both in mercury poisoning and in children with autism, but overall the clinical picture of mercurism—from any known form, dose, duration, or age of exposure—does not mimic that of autism. No case history has been encountered in which the differential diagnosis of these 2 disorders was a problem. Most important, no evidence yet brought forward indicates that children exposed to vaccines containing mercurials, or mercurials via any other route of exposure, have more autism than children with less or no such exposure.”Thimerosal and Autism?
Karin B. Nelson, Margaret L. Bauman
Pediatrics Mar 2003, 111 (3) 674-679; DOI: 10.1542/peds.111.3.674
As you can see, because the signs and symptoms of autism are similar to those of mercury poisoning, many people made the leap into thinking that autism was mercury poisoning. However, those who understand the biochemistry of mercury exposure at the levels contained in vaccines would know that it is biologically implausible for vaccines to cause toxicity, let alone autism. It is just simply too little an amount of mercury and in a form that is not toxic at those levels.
The best example I give my public health students is that the ethyl mercury in thimerosal is like ethanol in alcoholic drinks. On the other hand, methyl mercury from, say, continuous exposure to contaminated seafood, is like methanol in adulterated drinks. while ethanol can be dangerous in the long run if consumed in excess in things such as beer or wine. It's all about the chemistry.
For an even better understanding of mercury, this video from NileRed () explains the chemistry and characteristics of mercury. Listen closely to the difference in how mercury alone behaves and how it behaves with other substances. And note how the presenter notes when mercury is dangerous and when it is safe.
Today, the only childhood vaccine that contains thimerosal is the influenza vaccine given from multi-dose vials. However, most children receive the influenza vaccine from single-dose syringes. Even with the removal of thimerosal from childhood vaccines, the prevalence of autism has continued to increase. (Prevalence is the number of existing cases divided by the population at risk for a given unit of time. Incidence is the number of new cases divided by the population at risk for a given unit of time.) That increase in prevalence has been the result of better diagnosis of Autism Spectrum Disorder, better access to services, and better acceptance of Austistics in our society. If thimerosal in childhood vaccination truly had been the cause of autism, then the incidence and prevalence of autism would have declined precipitously in the last 20 years.