This mercury-containing compound, used as a vaccine preservative, is commonly used in lower-income countries — and deemed safe. The U.S. is now demanding that Gavi, the Vaccine Alliance stop using it.

Advisers to the Centers for Disease Control and Prevention recommended against flu shots containing the ingredient thimerosal. Why is the additive, safely used since the 1930s, being questioned again?

The CDC's Advisory Committee on Immunization Practices voted on the flu vaccine, raising concerns about a rarely used preservative. Medical groups worry this will "sow distrust" in vaccines.