Which vaccine is the first to protect against a disease that was once one of the most feared childhood infections?

Diphtheria was once a major killer of children, causing thick throat membranes, breathing problems, and heart complications. The diphtheria toxoid vaccine became one of the classic childhood vaccines of the 20th century and later formed part of combination shots such as DTP. Before vaccination, diphtheria outbreaks were common and often deadly. Mass immunization transformed it from a feared killer into a rare disease in many countries.