Vibrio alginolyticus is an important bacterial pathogen causing hemorrhagic disease and significant mortality in marine fish, including red drum, Sciaenops ocellatus (Linnaeus, 1766). This study evaluated the efficacy of an enteric-coated oral inactivated V. alginolyticus vaccine and optimized vaccination strategies to improve protection against vibriosis. A virulent strain (VA15) was selected for challenge experiments, with a median lethal dose (LD₅₀) of 2 × 10⁴ CFU fish-1. Juvenile red drum (14-16 g) were orally immunized with vaccine doses ranging from 10⁵ to 10⁷ CFU equivalent fish-1 to determine the optimal antigen concentration. Antibody titres increased significantly following vaccination and reached peak levels at 21 days post-immunization. As no significant difference was detected between the 10⁶ and 10⁷ CFU equivalent fish-1 groups, the 10⁶ CFU equivalent fish⁻¹ dose was selected for subsequent trials because it provided comparable immunogenicity while requiring less antigen. The protective efficacy of unencapsulated and enteric-coated vaccine formulations was subsequently evaluated using different booster schedules. Fish receiving the unencapsulated vaccine achieved a relative percent survival (RPS) of 44%, whereas enteric coating increased RPS to 54% following a single vaccination. Booster immunization further enhanced protection, with RPS values increasing to 61% and 78% when boosters were administered on days 14 and 21, respectively. These findings demonstrate that enteric coating improves oral vaccine delivery and that a booster vaccination administered 21 days after primary immunization provides the highest level of protection against V. alginolyticus infection. Although the study was conducted under laboratory conditions using a single bacterial strain, the results highlight the potential of enteric-coated oral vaccines as a practical disease management strategy for marine aquaculture.
