Evaluation of immune responses in cattle immunized with an experimentally developed inactivated aluminum potassium sulfate-adjuvanted Brucella abortus biovar 3 vaccine.

Brucellosis remains a critical public health and economic concern in Bangladesh due to its zoonotic transmission and severe reproductive losses in cattle. This study evaluated the safety, immunogenicity, and field effectiveness of an experimentally developed inactivated aluminum potassium sulfate (alum)-adjuvanted Brucella abortus biovar 3 vaccine prepared from a local strain using BALB/c mice for safety assessment and dairy cattle for field evaluation. A total of 1,570 dairy cattle from 30 farms across 6 regions were enrolled, of which 1,227 were vaccinated and 343 remained unvaccinated. Animals were monitored for 1 yr using serological assays, including Rose Bengal plate test and i-ELISA, molecular confirmation by Abortus, Melitensis, Ovis, Suis-Erythritol PCR, and cellular immune response assessment through delayed-type hypersensitivity testing and histopathology. Vaccinated cattle developed detectable antibody responses by 15 d postvaccination, with titers peaking at 90 d and remaining elevated compared with baseline and unvaccinated controls up to 180 d postvaccination. Delayed-type hypersensitivity responses and histopathological findings further supported vaccine-induced cellular immunity at antigen-injected sites. Field evaluation showed a marked reduction in seropositive abortion, with total abortions declining from 89 before vaccination to 33 after vaccination. During the postvaccination period, seropositive abortion occurred in 1 of 1,227 vaccinated cattle and 32 of 343 unvaccinated cattle. After accounting for farm-level clustering, vaccination remained strongly associated with reduced abortion risk. The cluster-robust Poisson model estimated an adjusted risk ratio of 0.0074, corresponding to an adjusted vaccine effectiveness of 99.26% (95% CI: 94.08%-99.91%). Mixed-effects logistic and farm-clustered generalized estimating equation models produced comparable estimates, supporting the robustness of the finding. No adverse effects were recorded in either mice or cattle, and vaccine sterility was confirmed by culture, indicating the absence of viable bacteria. These findings indicate that the locally developed inactivated alum-adjuvanted B. abortus vaccine is safe, immunogenic, and strongly associated with reduced seropositive abortion under farm-clustered field conditions, supporting its potential use as part of brucellosis control strategies in Bangladesh.