What is thimerosal?
Thimerosal is a chemical that contains a small amount of mercury and is used in some vaccines and other medicines to keep them from spoiling. It prevents bacteria and fungi from growing in vaccines that come in multi-dose vials so they remain effective until the last dose is used.
Is thimerosal in all vaccines?
No. Most vaccines used in Ontario do not contain thimerosal. Since 1994, all routine childhood vaccines have not contained thimerosal. Thimerosal is not added to single dose vaccines such as PentacelTM. The Hepatitis B vaccine that students get in Grade 7 and the flu shot offered to all Ontario residents each fall contain thimerosal since they come in multi-dose vials.
Does thimerosal cause autism or behaviour diseases?
The best available science to date has shown no link between vaccines containing thimerosal and autism or other behaviour disorders. International bodies, such as the World Health Organization, the U.S. Food and Drug Administration and the Institute of Medicine in the U.S., support this conclusion.
The National Advisory Committee on Immunization (NACI), a Canadian committee of experts on vaccines, has reviewed thimerosal and concluded that no link between autism or other behavioural diseases has ever been substantiated. Nevertheless, as a precautionary measure, NACI, as well as other international advisory bodies, has suggested minimizing the amount of thimerosal or eliminating its use in vaccines to reduce unnecessary exposure to mercury.
I have read that studies show thimerosal does cause neurological damage.
A study published in 2004 suggested thimerosal and a number of other chemicals may affect the development of the nervous system and lead to an increased risk of neurological disorders, such as autism. Health Canada found the study’s data did not support this. The study did not look at the effect of thimerosal on people but instead looked at cells in a laboratory test. It is not clear what this means for people. Many drugs have different effects in laboratory tests and in people. Other studies comparing children who received vaccines with thimerosal and children who received vaccines without thimerosal found the risk of autism was the same for both groups.
Are there studies that show no link between thimerosal and autism?
Yes. In May 2004, the Institute of Medicine (IOM), an impartial group of U.S. scientists concluded that, based on all the best studies done on this issue, thimerosal-containing vaccines do not cause autism. The group concluded that any links between these vaccines and autism are theoretical and that the causes of autism remain unknown.
Why is thimerosal used in the flu vaccine if other vaccines do not contain it?
The flu vaccine is provided to Toronto Public Health in multi-dose vials and thimerosal is added in the manufacturing process to keep the vaccine from spoiling once a vial is opened. Thimerosal will continue to be used in multi-dose vaccines until an effective alternative is found.
Could another preservative be used in multi-dose vaccines like the flu vaccine?
Yes. Pharmaceutical companies are actively working on alternatives to thimerosal. If alternatives are found, they will need to be tested in clinical trials to evaluate their safety and effectiveness as preservatives.
Are children at an increased risk of developing neurological disorders like autism or Attention Deficit Hyperactivity Disorder (ADHD) if they received a flu shot?
No. The best available science to date shows there is no link between vaccines and autism or ADHD.
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