Based on the current immunization schedule, for most infants, either of these approaches should allow completion of the necessary three doses of vaccine by 18 months of age. Until thimerosal-free vaccine is available, immunization for the small, prematurely born infant should be deferred until the infant reaches a size and development that corresponds to that of a term infant at birth. Hepatitis B vaccine which does not contain thimerosal is expected to be made available in the near future. Thimerosal is an effective preservative that contains mercury. It is the most widely used preservative in vaccines and has been used in some vaccines and other products since the 1930s.
Thimerosal prevents bacterial contamination of multi dose vaccine vials when they are opened.
Concern has been raised about the very small amount of mercury in some vaccines but no data or evidence indicates any harm caused by the extent of exposure that some infants and children may have encountered in following the routine immunization schedule. Children are not getting toxic quantities of mercury from vaccines. Exposure to mercury is of concern and, when possible should be avoided. The AAP and the Public Health Service agree that thimerosal should be reduced or eliminated in vaccines to make already safe vaccines even safer, and vaccine manufacturers are working toward this goal. Recent review of FDA guidelines for mercury exposure and the mercury content of vaccines indicate that some children, depending on which vaccines they receive and the timing of those vaccines, may be exposed to cumulative amounts of mercury close to the upper limit of safety ranges of the guidelines.