If the MMR vaccine doesn't contain mercury, then why is there so much publicity about mercury in vaccines causing autism?

Mercury is a metal that occurs naturally and is found everywhere in the environment. There are different types of mercury. Thimerosal, a preservative used in certain vaccines, contains approximately 49% ethylmercury. Here's how the CDC describes ehtylmercury:

Methylmercury is of particular concern because it can build up in certain edible freshwater and saltwater fish and marine mammals to levels that are many times greater than levels in the surrounding water. Thimerosal is an organic mercury compound that is metabolized to ethylmercury and thiosalicylate.

Since mercury is known to cause neurological damage, the case has been made that vaccinations containing even trace amounts of ehtylmercury could be at least partially responsible for the growing number of autism diagnoses. Studies looking into this question have come up with apparently contradictory answers.

The National Immunization Program of the CDC offers a great deal of information about thimerosal and vaccines, including information about the availability of thimerosal-free vaccines.