Efficacy of bovine brucellosis vaccines: Conceptions, challenges and meta‐analysis. Response to the Letter to the Editor concerning “Efficacy of Brucella abortus S19 and RB51 vaccine strains: A systematic review and meta‐analysis” by Blasco et al (Transbound Emerg Dis; 2021: https://doi.org/10.1111/tbed.14259)