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.

Flu shots that have more than one dose of vaccine contain thimerosal; other versions don’t have it. HHS said Wednesday that vaccine makers have said they have the capacity to replace the shots that do contain thimerosal so that vaccine supplies aren’t interrupted.