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Thimerosal has been the subject of several studies and has a long record of safe and effective use preventing bacterial and fungal contamination of vaccines, with no ill effects established other than minor local reactions at the site of injection. You don't need to worry about it. But there are some flu shots free of Thimerosal. http://www.fda.gov/BiologicsBloodVaccines/SafetyAvailability/VaccineSafety/UCM096228#t3