What is thimerosal, does it have side effects and why is RFK Jr. removing the controversial preservative from the flu shot? Here's what to know.

The mercury-based ingredient has prompted concerns from some organizations, although the CDC has said there is no evidence it causes harm.

The federal government is recommending flu vaccines remove the preservative thimerosal after Health and Human Services Secretary Robert F. Kennedy Jr. on Wednesday officially adopted the recommendation of a Centers for Disease Control and Prevention (CDC) advisory panel.