The most trustworthy/neutral source I could find on this is a 2010 USA Today article on flu myths
http://usatoday30.usatoday.com/yourlife/health/medical/coldflu/2010-11-15-myths-flu_N.htm
Myth 5: Flu shots contain methyl mercury and other toxic chemicals.
Although there's no evidence that the ethyl mercury-based preservative thimerosal causes harm, vaccine makers responded to public concerns in 2001 and stopped using it in most vaccines.
Neither flu shots in individual-dose containers or the FluMist nasal spray contain thimerosal. Flu vaccine kept in multi-dose vials do use thimerosal as a preservative, to prevent the growth of fungus or other potentially dangerous germs, Bergen says. Patients can ask for the thimerosal-free versions.
But scientists note that all mercury is not the same.
Thimerosal contains
ethyl mercury, which has not been shown to cause harm, rather than
methyl mercury, the type that can cause brain damage, Offit says. While most laypeople don't pay attention to such differences, they're important. Consider the huge difference between ethyl alcohol or drinking alcohol, found in wine and beer and methyl alcohol, or wood alcohol, which can cause blindness.
There's also no data to prove that thimerosal causes autism, Offit says. In fact, seven studies now refute that idea.
Gunter notes that flu shots don't use aluminum, which is used in other vaccines as an "adjuvant" to stimulate a stronger immune response.
While many medications and consumer products have trace levels of chemicals, so do our bodies, Offit says. Breast milk has more mercury contamination than vaccines. So does infant formula. But vaccines, like breast milk, play a vital role in keeping infants healthy.
"If you have zero tolerance for mercury, you have to move to another planet," Offit says. "We all have mercury and formaldehyde and aluminum in our bodies. Vaccines don't add to what we normally encounter every day."