Preservatives are used in vaccines to prevent or kill any growth of bacteria, fungi, or other microorganisms that might have contaminated the vaccine vial. Microorganisms are able to get into the vials that are punctured more than once with a needle. Multiple punctures only occur in vaccine vials that are labeled as multi-dose vials and are intended to be used with different needles more than one time.
Due to tragic events in vaccine history, such as the infection of children with deadly bacteria after injection with multi-dose vaccines, the United States Code of Federal Regulations (the CFR) required preservatives be added to multi-dose vials of vaccines. Because of many incidences of infections due to vaccinations, preservatives have been required in multi-dose vaccines since the 1930’s.
Since then, thimerosal has been the most widely used preservative in multi-dose vaccines. It has a long record of safety and has been proven effective at preventing bacterial and fungal contamination of vaccines. However, over the past several years, thimerosal has become a target for controversy due to its mercury concentration and concern for safety.