A new slate of vaccine advisers to the US Centers for Disease Control and Prevention voted Thursday to recommend that Americans receive influenza vaccines that are free of thimerosal even though there is no evidence of harm from the preservative.

Thimerosal has been at the heart of Health and Human Services Secretary Robert F. Kennedy Jr.’s antivaccine activism for 20 years. Here’s what to know about the vaccine preservative and its removal from flu vaccines.

Health experts were shocked to see that thimerosal, a preservative that has largely been phased out of U.S. vaccines, was on the ACIP agenda.