This product is not approved for use before 6 weeks of age because of decreased response to the Hib component when administered at this age. If thimerosal-free vaccine is not available, hepatitis B virus vaccination should be initiated at 6 months of age. 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.