There is no proof that ethyl mercury is more or less harmless then methyl mercury.
From the NATIONAL INSTITUTES OF HEALTH US DEPT OF HHS.
http://www.niaid.nih.gov/factsheets/thimerosal.htmSince the 1930's, thimerosal has been added to some vaccines and other products because it is effective in killing bacteria and in preventing bacterial contamination, particularly in multi-dose containers.
When thimerosal is degraded or metabolized, one product is ethyl mercury, another organic derivative of mercury. Not much is known about the effects of thimerosal exposure on humans and how this compares to methyl mercury exposure. The only known (admitted) side-effects of receiving low doses of thimerosal in vaccines have been minor reactions such as redness and swelling at the injection site.
In July 1999, U.S. Department of Health and Human Service agencies, The American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure and to reduce exposure to mercury from all sources. Today, all routinely recommended licensed pediatric vaccines currently being manufactured for the U.S. market are either thimerosal-free or contain markedly reduced amounts of thimerosal. However, thimerosal remains in some vaccines given to adults and adolescents, as well as some pediatric vaccines not on the Recommended Childhood Immunization Schedule. Thimerosal is a common preservative found in vaccines used outside the United States.
The decision to move toward reduced or eliminated thimerosal in vaccines was based on the various Federal guidelines for methyl mercury exposure and the assumption that the health risks from methyl and ethyl mercury were the same. Methyl mercury exposure is primarily through fish consumption. People who regularly eat mercury-contaminated fish can accumulate methyl mercury in their body over time. Some of this methyl mercury may be passed from the mother to the fetus before birth and to infants through breast milk. The fetus is sensitive to damage by this exposure.
Prior to the removal of thimerosal from childhood recommended vaccines, infants were exposed to ethyl mercury by intramuscular injection during vaccination, not by ingestion. Furthermore, infants received thimerosal from childhood vaccines that were administered days or months apart. In contrast, methyl mercury exposure, primarily from foods, tends to occur over a longer sustained period of time.
More research is needed to determine if the guidelines for methyl mercury exposure are also appropriate guidelines for thimerosal. Additionally, guidelines for maximal levels for short-term exposure need to be established.Many scientists with and without ties to the industry along with many pediatricians believe that thimerosal (a form of mercury) is a factor in autism - or at best say we don't know.