Read the inserts to the vaccine. "Thimerosal, a mercury derivative, is not used in the manufacturing process for the single dose presentations; therefore these products contain no preservative. The multi-dose presentation contains thimerosal, added as a preservative; each 0.5 mL dose contains 24.5 mcg of mercury. " From: http://www.fda.gov/downloads/BiologicsBloodVaccines/Vaccines...