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.

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.

The vast majority of research has failed to show any link between thimerosal and autism or other neurological issues.