Some vaccines contain thimerosal. It is a preservative with ethyl mercury. Let us start by saying that this has never harmed any recipients of a vaccine, and there is no credible research to support such. Anyone who claims otherwise is either misinformed or attempting to misinform. That being said, if thimerosal really makes you nervous, you’ll be happy to hear that the MMR vaccine contains no traces of it. Even though mercury in volumes so low as in thimerosal is harmless, you can have that little extra bit of peace of mind knowing that your MMR vaccine is mercury-free.
There are countless studies and resources confirming the validity of the statement that MMR is free of mercury. If you see someone citing a study that says otherwise, be sure to check its credibility, as it is likely a phony study used to sway opinion. Be sure to get your information from credible studies only.