CDC advisors have recommended against flu shots containing thimerosal, an ingredient used safely since the 1930s. The recommendation raises questions about why this established additive is being re-evaluated. Thimerosal is a mercury-based preservative that prevents bacterial contamination in multi-dose vaccine vials. While it has been deemed safe by numerous studies, concerns about mercury exposure persist among some groups. The CDC's advisory committee likely considered these ongoing concerns and the availability of thimerosal-free alternatives in their recommendation.