Veterinary Vaccine Adjuvants: Past, Present, and Future

Vaccine adjuvants enhance immune responses and improve the effectiveness of veterinary vaccines. However, their performance and safety vary across animal species, vaccine formulations, and production systems. This narrative review examines the historical development, mechanisms, applications, safety, and regulatory considerations of major veterinary adjuvant classes. Aluminum salts are widely used because of their affordability and ability to enhance antibody responses, although their effects on cellular immunity may be limited. Oil-based emulsions can provide strong and durable immune responses, particularly in inactivated vaccines for livestock and poultry, but injection-site reactions and formulation stability remain important concerns. Saponin-based systems can stimulate both antibody and cellular immunity, with applications in equine and canine vaccines. Polymeric nanoparticles and pattern-recognition receptor agonists offer promising approaches for mucosal vaccination and targeted immune activation, although further validation in veterinary species is needed. Adjuvant selection must consider antigen properties, species-specific immune responses, age, administration route, protective efficacy, manufacturing feasibility, and regulatory requirements. Future research should prioritize comparative studies in target species, predictive biomarkers, improved vaccine stability, and evaluation of complete antigen–adjuvant formulations. These considerations may support the development of safer, more effective, and practical veterinary vaccines.