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.

The preservative thimerosal has been removed from all but a few vaccines, and studies have confirmed no link to neurodevelopmental issues including autism. So public health experts were puzzled when it appeared on the agenda for this week’s meeting of outside vaccine advisers to the US Centers for Disease Control and Prevention.

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.