Thimerosal is a mercury-containing compound that has been utilized as a preservative in various medical and biological products, including certain vaccines, since the 1930s. Its primary function is to prevent the growth of harmful microbes, such as bacteria and fungi, that could contaminate multi-dose vaccine vials.
Historical Use and Purpose
Thimerosal was historically incorporated into vaccines as an antimicrobial preservative. It was important for maintaining sterility in multi-dose vaccine vials after they were opened and used for multiple administrations.
Each time a syringe needle enters a multi-dose vial, there is a risk of introducing bacteria or fungi. Thimerosal inhibits the growth of these microorganisms, preventing contamination that could lead to severe reactions, illness, or even death. This preservative allowed for the safe and practical use of larger vaccine vials for multiple patients, contributing to more efficient immunization campaigns.
Addressing Safety Concerns and Scientific Findings
Public concerns about thimerosal arose, particularly regarding a potential link to autism and other neurodevelopmental disorders. This led to extensive scientific investigation by major health organizations globally, including the Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), and the Institute of Medicine (IOM). These studies consistently found no causal link between thimerosal in vaccines and autism or other developmental problems.
Thimerosal’s chemical composition includes ethylmercury, which distinguishes it from methylmercury, a different form of mercury found in environmental sources like certain fish. Ethylmercury is processed and eliminated from the human body much more quickly than methylmercury, making it less likely to accumulate and cause harm. Studies have shown that ethylmercury breaks down and is excreted rapidly, primarily through stool, differing significantly from methylmercury’s slower excretion.
Research indicates mercury from thimerosal-containing vaccines is cleared from an infant’s blood faster than predicted for methylmercury. The clinical symptoms of mercury poisoning also differ significantly from autism characteristics. Despite thimerosal’s removal from most childhood vaccines, autism rates continued to increase, further supporting the lack of association.
Current Use in Vaccines
As a precautionary measure, thimerosal has been removed from most childhood vaccines in the United States and Europe since 2001, not due to any proven safety concerns. This decision aimed to reduce an infant’s total mercury exposure, considering other environmental sources. All routinely recommended childhood vaccines for children aged six years and younger in the U.S. are now available in thimerosal-free formulations.
Thimerosal may still be present in trace amounts in some vaccines during early manufacturing stages, though very little remains in the final product after purification. It is still found in some multi-dose vials of influenza (flu) vaccines, which account for a small percentage of all flu vaccines administered in the U.S. Single-dose, thimerosal-free flu vaccine options are widely available for both children and adults.