Brucellosis is of worldwide economic and public health importance. Heifer vaccination with live attenuated <i>Brucella abortus</i> strain 19 (S19) is the cornerstone of control in low- and middle-income countries. Antibody persistence induced by S19 is directly correlated with the number of colony-forming units (CFU) per dose. There are two vaccination methods: a ‘high’ dose (5–8 × 10¹⁰ ...