First of all, it's thimerosal. According to the FDA, it's been used since the 1930's but most vaccines now have little or none.
No harmful effects have been reported from thimerosal at doses used in vaccines, except for minor reactions like redness and swelling at the injection site. However, in July 1999, the Public Health Service (PHS) agencies, the American Academy of Pediatrics (AAP), and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure.Today, all routinely recommended pediatric vaccines manufactured for the U.S. market contain no thimerosal or only trace amounts.
Did you know that the mercurochrome our parents used to paint our boo-boos with also contained mercury?