Thimerosal
In recent years, various federal agencies have been addressing the health risks of mercury, which is found in the environment, in food and in household products. Although no harmful effects have been reported from thimerosal at doses that were used in vaccines, the PHS agencies, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines to make already safe vaccines even safer. Some reports have indicated that the mercury containing thimerosal used in vaccines has caused problems for children. These reports propose that a build up of mercury during the first 6-12 months of life can cause severe damage including autism.
What is Thimerosal?
Thimerosal is a preservative used in vaccines to prevent spoiling, to inactivate bacteria used to make certain vaccines, and to prevent bacterial contamination in the finished vaccine.What are preservatives and why are they added to vaccines?
Preservatives are compounds that kill or prevent the growth of microorganisms, such as bacteria or fungi. They are used in vaccines to prevent bacterial or fungal growth in the event that the vaccine is accidentally contaminated, as might occur with repeated puncture of multi-dose vials. Vaccines, both in the United States and throughout other parts of the world, are commonly packaged in multi-dose vials. In some cases, preservatives are added during manufacture to prevent microbial growth; with changes in manufacturing technology, however, the need to add preservatives during the manufacturing process has decreased markedly.
Preservatives have been used in vaccines for over 70 years. The requirement for a preservative in multi-dose, multi-entry vials was placed into the Code of Federal Regulations (21 CFR 610.15) in January 1968. There are exceptions to this requirement for preservative, primarily involving the live-attenuated viral vaccines.
The general need for preservatives in multi-dose vials has been attested to by a number of examples of multi-dose vials being formulated without preservatives becoming contaminated during use, and causing the death of vaccine recipients; cf. the Narrative Section on Thimerosal.
Thimerosal History
Thimerosal is a preservative that has been used in some vaccines since the 1930's, when it was first introduced by Eli Lilly Company. It is 49.6% mercury by weight and is metabolized or degraded into ethyl mercury and thiosalicylate. At concentrations found in vaccines, it meets the requirements for a preservative as set forth by the United States Pharmacopeia; that is, it kills the specified challenge organisms and is able to prevent the growth of the challenge fungi. Prior to its introduction in the 1930's, data were available in several animal species and humans providing evidence for its safety and effectiveness as a preservative. Since then, thimerosal has a long record of safe and effective use preventing bacterial and fungal contamination of vaccines, with no ill effects established other than minor local reactions at the site of injection.
While the use of mercury-containing preservatives has declined in recent years with the development of new products formulated with alternative or no preservatives, thimerosal is still used in certain antivenins, skin test antigens, and ophthalmic and nasal products, in addition to certain vaccines recommended for adults or older children. As a vaccine preservative it is used in concentrations of 0.003% to 0.01%. A vaccine containing 0.01% thimerosal as a preservative contains 50 micrograms of thimerosal per 0.5 ml dose or approximately 25 micrograms of mercury per 0.5 mL dose.
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