Thimerosal, composed of ethyl mercury, is a compound used as a preservative in vaccines. Ethyl mercury is quickly eliminated by the body and DOES NOT cause mercury poisoning. Mercury poisoning is caused by methyl mercury, a compound that does build up in the body and can result in damage to the nervous system and brain. So, while the two types of mercury sound similar, they are very different. Thimerosal has not been shown to be toxic. But thimerosal has been removed from vaccines whenever possible. Currently, flu vaccine is the only childhood vaccine that contains thimerosal, and there are typically more than an adequate number of thimerosal-free doses of flu vaccine produced for children each year. Also, even when it was present in vaccines, the total amount of thimerosal containing mercury received by an infant was less than half the amount of mercury a baby receives from its mother if breast fed for six months. When looking only at flu shots, breastfed babies receive 25 times more mercury in the breast milk than they would from a flu shot. As of 2019, approximately 85% of the flu vaccine produced is thimerosal-free.