Thimerosal is added to all multi-dose vaccines for adults.   Thimerosal is a mercury organo-compound.

   Multi-dose vaccines are packaged in a way that can be dispensed more easily (and stored for longer periods) for large vaccination programs.

//www.cdc.gov/vaccinesafety/concerns/thimerosal/index.html

Note that in 1999 the CDC suggested that the use of thimerosal should be reduced or eliminated as a "precautionary measure".   Note that this is around the time when the full sequencing of DNA was completed for the first time.

The big issue with thimerosal lies with people who metabolically have difficulty eliminating it from their bodies.   People with "heat shock protein" gene abnormalities would fall in this category.   The number of people with heat shock protein abnormalities is significant. 

NB:  One clue that you might have heat shock protein issues is if you have more trouble in high temperature/humid environments than most people.   There are other clues too.