Thimerosal in Vaccines and Autism

Questions and Answers

1. What is thimerosal?
2. Is thimerosal in all vaccines?
3. Does thimerosal cause autism or other neurodevelopmental disorders?
4. Is the use of thimerosal in vaccines a health concern?
5. What the difference between a single dose and a multi-dose vaccine?
6. Why is thimerosal used in the flu vaccine if other vaccines do not contain it? Are there flu vaccines available that don’t contain thimerosal?
7. Could another preservative be used in multi-does vaccines like the flu vaccine?

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Q1. What is thimerosal?

Thimerosal is a mercury-based preservative used in the manufacturing process of multidose vaccines.

Thimerosal helps: 

• to prevent the growth of bacteria and fungi in vaccines. 
• to stabilize the vaccine so that it remains effective over time.

The use of thimerosal is necessary to prevent infections at the vaccination site.  Infection at the site of the injection can lead to bloodstream infections and abscesses.

Q2. Is thimerosal in all vaccines?

No. Most vaccines licensed in Canada do not contain thimerosal.
The NACI updated statement on thimerosal from July 2007 has a table which lists the thimerosal content of vaccines marketed in Canada.

The influenza vaccine and most hepatitis B vaccines are multi-dose vaccines, which contain thimerosal as a preservative.

For immunization of infants against hepatitis B, parents or guardians in some provinces and territories have the choice of a thimerosal-free vaccine.

Q3. Does thimerosal cause autism or other neurodevelopmental disorders? 

No. The best available science to date has shown that there is no link between vaccines containing thimerosal and autism or other behaviour disorders.

The National Advisory Committee on Immunization (NACI) has reviewed the safety of thimerosal and concluded that the alleged adverse health effect from thimerosal in vaccines has never been substantiated.

International bodies, such as the World Health Organization (WHO), the U.S. Food and Drug Administration (U.S. FDA) and the Institute of Medicine in the U.S., share this opinion.

Q4. Is the use of thimerosal in vaccines a health concern?

No. There is no safety reason to avoid thimerosal-containing vaccines.
The National Advisory Committee(NACI) has thoroughly reviewed the evidence and indicated that “There is no legitimate safety reason to avoid the use of thimerosal-containing products for children or older individuals, including pregnant women.”

Nevertheless, NACI has recommended a long-term goal of removing thimersol from vaccines, provided that safe alternatives to this preservative can be found. This will help to reduce unnecessary environmental exposure to mercury.

Thimerosal is a mercury-based preservative. In large concentrations, or over extended periods of exposure, mercury can cause damage to the brain and the kidneys. The organic form, methylmercury, is the main known cause of neurological damage. The sources of methylmercury are in the environment, with fish as a food source being of greatest concern. Maximum tolerable levels for daily intake of methylmercury relative to body weight have been established. 

Thimerosal contains ethylmercury, not methylmercury. The original concern regarding thimerosal in vaccines was a purely theoretical one, brought to attention in the 1990s based on the total amount that would be received in the infant schedule as recommended in the United States at that time. The recommended maximum levels for methylmercury exposure were used because no such guidelines existed for ethylmercury.

The assumption was that methylmercury and ethylmercury are handled in the same way by humans. This has been shown to be false. Ethylmercury is eliminated much more quickly and is less likely to reach toxic levels in the blood than methylmercury.  

The levels of ethylmercury in vaccines are minute and have not been shown to cause harm. Millions of doses of thimerosol-containing vaccines have been administered to patients since the 1930’s with no common or serious adverse events observed.

Q5. What the difference between a single dose and a multi-dose vaccine?

A single dose vaccine is one that is stored in a single vial that is disposed of after the one dose is given to a person.

A multi-dose vaccine is one that’s stored in a vial which contains 10 doses of the vaccine. One vial of a multi-dose vaccine can be used to immunize up to 10 different people.

Q6. Why is thimerosal used in the flu vaccine if other vaccines do not contain it? Are there flu vaccines available that don’t contain thimerosal?

The flu vaccine is generally marketed in a multi-dose vial and thimerosal is added to the manufacturing process to maintain sterility of the vaccine. Thimerosal also has a stabilizing effect in the vaccines, ensuring that they are effective.

There are two thimerosal-free influenza vaccines approved for sale in Canada, however they are not currently available for publicly funded programs. One of these vaccines (Influvac from Solvay) is not currently approved for use in individuals under 18 years of age and both are available only in the private market.

Immunization programs are implemented by provincial/territorial governments, and each is responsible for assessing and meeting the vaccination needs of their particular jurisdiction. Most jurisdictions prefer to use the multi-dose format (10 dose vials) and these require a preservative (thimerosal).

Thimerosal will continue to be used in multi-dose vaccines until a safe alternative is found.

Q7.Could another preservative be used in multi-does vaccines like the flu vaccine?

Yes. Pharmaceutical companies are actively working on alternatives to thimerosal as a preservative. If alternatives are used, they will need to be tested in clinical trials to evaluate their safety and effectiveness as preservatives. In some initial tests, alternative stabilizers have actually made the vaccines less effective.

Additional Information

For additional information on thimerosal and autism, please visit: 

• NACI statement on thimerosal – July 2007
• Autism
• Exposure to Thimerosal in Vaccines used in Canadian Infant Immunization Programs, with respect to risk of neurological disorders