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.

It is rarely included in vaccines in the United States. The flu shot was the sole routinely recommended vaccine to include the preservative. And roughly 96 percent of all flu shots in the United States were thimerosal-free during the last influenza season, according to data from the C.D.C.

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.