Health and Human Services Secretary Robert F. Kennedy, Jr. is once again warning about the supposed dangers posed by mercury-containing vaccines. "I’m proud to finally deliver on a long-overdue promise: protecting our most vulnerable from unnecessary mercury exposure," he tweeted in early August.

Kennedy was celebrating the removal of the preservative thimerosal from a small fraction of seasonal flu vaccines. The problem? He bungled basic chemistry in his rush to (once again) badmouth immunization. Let's take a look.