Thimerosal, a very effective preservative, has been used to prevent bacterial contamination in vaccine vials since the 1930s. It contains a type of mercury known as ethylmercury, which is different from the type of mercury found in fish and seafood (methylmercury). At very high levels, methylmercury can be toxic to people, especially to the neurological development of infants.
In recent years, several large scientific studies have determined that thimerosal in vaccines does not lead to neurologic problems, such as autism. Nonetheless, because we generally try to reduce people’s exposure to mercury when possible, vaccine manufacturers have voluntarily changed their production methods to produce vaccines that are now free of thimerosal or have only trace amounts. They have done this because it is possible to do, not because there was any evidence that the thimerosal was harmful.
Comprehensive information about thimerosal in vaccines is available from the U.S. Food and Drug Administration at www.fda.gov/BiologicsBloodVaccines/SafetyAvailability/VaccineSafety/ucm096228.htm. The Vaccine Education Center at Children’s Hospital of Philadelphia also has extensive information about thimerosal and other vaccine ingredients at www.chop.edu/centers-programs/vaccine-education-center.