When was the first plague vaccine developed?

Waldemar Haffkine developed an early plague vaccine in 1897 after working during a major plague outbreak in India. His vaccine was tested in people and used in large-scale public health efforts, making it one of the earliest mass vaccination campaigns against a bacterial disease. Plague is caused by Yersinia pestis and has historically been associated with devastating epidemics. Although the vaccine was imperfect, it was a major milestone in preventive medicine.