Thimerosal and Autism

Vials and Syringe

Although there are a number of studies that refute a connection, some parents, educators, and scientists believe that the vaccine preserving agent, thimerosal, is linked to increasing autism rates. It's essential for parents to understand the details of this controversy in order to make informed decisions about their children's health.

Understanding Thimerosal

Thimerosal is an effective vaccine preservative that used to be used in children's vaccines, such as measles, mumps, and rubella (MMR). The preservative allows vaccine manufacturers to create less costly multi-dose containers of medication, keeping the vaccine fresh and free from bacterial contamination despite repeated contact with needles.

Thimerosal Vaccine Controversy

A review of the literature published in the journal Clinical Infectious Diseases asserts that the origin of the thimerosal vaccine controversy lies in a conservative recommendation by the American Academy of Pediatrics (AAP) in 1999. The AAP noted that in the absence of studies proving its safety, thimerosal should not be an ingredient in children's vaccines.

Thimerosal No Longer in Most Vaccines

In 1999, the FDA also recommended removing thimerosal as an ingredient in childhood vaccines. By 2003, the last of the thimerosal-containing vaccines had expired.

According to the Centers for Disease Control and Prevention (CDC), thimerosal is no longer used in the typical childhood vaccines, but it is still an ingredient in the influenza vaccine or "flu shot."

Mercury and Thimerosal

A large part of the vaccine controversy comes from the fact that thimerosal contains mercury, which the Environmental Protection Agency (EPA) describes as "neurotoxicant." This means that exposure to some forms of mercury, namely methylmercury, can cause neurological problems, especially in sensitive groups like developing fetuses, infants, and children.

The Food and Drug Administration (FDA) reports that thimerosal is made up of 50% mercury.

Type of Mercury in Thimerosal

It is important to note, however, that the form of mercury in thimerosal is ethylmercury, while the type of mercury associated with neurological problems is methylmercury. According to the World Health Organization (WHO), there are some significant differences between these mercury compounds:

  • Despite the similar names, ethylmercury and methylmercury are very different compounds. As an analogy, it's helpful to consider the difference between ethanol, the type of alcohol in beverages, and methanol or wood alcohol, which is extremely toxic to humans.
  • The half-life, or the time it takes for the body to break down or flush away half of the chemical, for ethylmercury is a few days. The half-life of methylmercury is over a month and a half.
  • As the body processes ethylmercury, it excretes it into the gut where it will eventually leave the body. However, methylmercury does not get excreted in this way and is stored in fat cells.

Mercury and Autism

There is also controversy about whether mercury exposure is a cause of autism; a number of studies have shown that the symptoms of mercury poisoning during infancy are very similar to those of autism. According a 2008 analysis of this issue in The International Journal of Risk and Safety in Medicine, it's impossible to confirm a link between mercury and autism while still using ethical research practices. However, the authors concluded that the likelihood of a link is high, and it's a wise choice to take precautions to avoid exposure.

Does Thimerosal Cause Autism?

For years, researchers have been examining whether there is a link between autism and the mercury in thimerosal. Initially, the vast majority of studies have found no proof of a connection, but they were unable to completely disprove the idea.

Later, large population-based studies helped show that thimerosal had no effect on autism rates. A few smaller studies have questioned whether some groups may be vulnerable to this type of environmental contamination. It can help parents to look over the research on both sides of this issue.

Studies Showing a Possible Link

Most studies have concluded that there is no association between thimerosal and ASD, but there are a few that suggest there could be a link:

  • A 2013 study published in the Journal of Toxicology found that thimerosal affected the development of immune cells in some children with autism, as well as some of the twins and siblings of people with autism. Approximately 30% of those families tested showed this vulnerability.
  • A 2014 study published in Toxicological Sciences examined the effect of thimerosal on the brains of young rats. Rats given a thimerosal dose 20 times that of the amount received in vaccines developed behaviors consistent with autism.
  • A 2012 study published in the journal Neurochemical Research found that infant rats exposed to high levels of thimerosal changed the amino acid balance in the brains of the rats, even weeks after the injections.

Studies Indicating No Link

There are dozens of studies refuting the idea that there may be a link between autism and thimerosal. The following large-scale studies are among the most conclusive:

  • A landmark Danish study publish in the Journal of the American Medical Association in 2003 found that in more than 1,200 children studied, there was no causal relationship between being given thimerosal-containing vaccines during infancy and later development of autism.
  • The journal Pediatrics published the results of a 2004 study of more than 14,000 children who were given thimerosal-containing vaccines. The children were no more likely to develop autism than their peers.
  • A 2014 meta-analysis in the journal Vaccine examined the current research in this area, looking at over 1,250,000 children. It concluded that there was no causal link between thimerosal and vaccines.

Tips for Avoiding Thimerosal

Although the majority of researchers have found that thimerosal is safe and does not cause autism, many parents still worry about exposing their children to this chemical. If you're hoping to avoid thimerosal, keep these tips in mind:

  • If you are getting a flu shot for your child, request one that does not contain thimerosal. Some offices have single-use vials that don't contain the preservative, and many doctors offer the thimerosal-free nasal vaccine for kids over the age of two.
  • If you are pregnant or nursing, you can choose a vaccine that doesn't contain thimerosal. The FDA keeps a current list of the thimerosal content in all vaccines. Reference or print this list and ask your doctor for vaccines without the preservative.
  • If you are pregnant and are Rh-negative, your doctor may give you Rhogam to prevent negative interactions between your blood and your baby's blood. Some types of Rhogam contain a thimerosal preservative, but some do not. The FDA offers a complete list for your reference.
  • Certain types of anti-venom for spider and snake bites may also contain thimerosal. However, avoiding the preservative should not be a primary concern when dealing with these emergency situations.

Remember, thimerosal is not in the routine vaccines given to children. You do not need to refuse these vaccines to minimize exposure to the preservative.

Knowledgeable Parents Are Powerful Advocates

No matter what you believe about thimerosal and its relationship to autism, educating yourself about this issue can help assuage your fears and give you the tools you need to make informed decisions about your child's medical care. A knowledgeable parent is a powerful advocate for her child.

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