Thimerosol is a preservative that is used in vaccines to prevent growth of harmful bacteria. While thimerosol has been in use in vaccines since the 1930s, some are concerned about the preservative because it contains mercury. For this reason, thimerosol is being used less often in newer vaccines, and has not been used in children’s vaccines since 2001.
Still, there is little reason to fear thimerosol, as the mercury it contains is of a different variety than the type that can accumulate in fish and cause harm in humans. According to the Centers for Disease Control and Prevention (CDC), the mercury contained in thimerosol does not remain in the body, and therefore is not likely to cause illness.