​​​Miriam E.Tucker

Mercury in Vaccines
 (Biotechnology Newswatch, September 6, 1999)

Vaccine manufacturers are scrambling to find a replacement for thimerosal, a mercury-containing preservative that has been used in some childhood vaccines since the 1930's. 
While there is no evidence of harm done to a child from thimerosal in a vaccine (aside from the rare hypersensitivity reaction), the U.S. Public Health Service and the American Academy of Pediatrics have now requested that manufacturers reduce or eliminate the use of the compound from existing and future vaccines whenever possible. 
This move was triggered by a recent government analysis which found that with the increased number of vaccines now being given to infants, some could be exposed to a cumulative amount of mercury that exceeds the safety guidelines for methylmercury set by the Environmental Protection Agency. 
Methylmercury in high doses is a developmental neurotoxin in humans. However, the mercury in thimerosal is ethyl, not methyl, and the exposure levels are well within the safety limits issued by the Food and Drug Administration and the Centers for Disease Control and Prevention. Furthermore, all those thresholds were largely based on studies of populations with high mercury exposure from fish consumption, a very different type and duration of exposure from that of immunization.
Vaccine manufacturers are attempting to comply, but they face some hurdles. Federal law requires the use of a preservative in most vaccines packaged in multidose containers, which comprise the bulk of the U.S. and worldwide vaccine markets.  Single-dose packs don't require preservatives, but many contain thimerosal anyway because it is much easier and cheaper to fill all the containers from the same batch of vaccine. 
Manufacturers expect to make thimerosal-free single-dose vials available soon, but it will take a lot longer to develop multidose packs with new preservatives. The FDA will require extensive data on the new compounds, inlcuding validation of their bacteriostatic and bacteriocidal properties, and proof of compatibility with other vaccine components. 
The FDA has said that they will expedite these reviews. Some of the newer vaccines already contain an alternative preservative called 2-phenoxyethanol, but experts believe that it is not as effective as thimerosal.  In cases where it is not possible to remove thimerosal, the FDA will consider approving vaccines with smaller amounts of the preservative. 
Meanwhile, many in the medical community are now concerned about how all this will impact the already-damaged public perception about vaccine safety. ###