Post-Vaccination Seromonitoring of Cattle and Buffalo Calves Following Brucella abortus S19 Vaccination: A Multi-Region and Multi-Time Point Study Across India (2021–2023)

Background: Brucella abortus S19 vaccination forms the cornerstone of the national brucellosis control program. Monitoring post-vaccination immune responses is crucial to assess vaccine performance across diverse regions and species. Methods: Two-phase nationwide seromonitoring study was conducted during 2021–2022 (Phase I) and 2022–2023 (Phase II) across seven geographical regions. A total of 51,759 serum samples were collected from S19-vaccinated cattle and buffalo calves of 4–8 months at four-days post-vaccination (DPV) intervals. Samples were analyzed using an in-house developed protein G-based indirect ELISA, and seroconversion rates were calculated. Statistical comparisons were performed, and spatial heterogeneity was mapped. Results: The overall seroconversion rate was 70.96%, with Phase I showing a higher seropositivity (76.05%) than Phase II (65.66%). Regionally, the Western (81.8%) and Southern (80.77%) regions showed the highest efficacy, while the Central region had the lowest (65.89%). Antibody kinetics peaked at 21–45 DPV (73.14%) and declined gradually to 43.27% by 91–120 DPV. A clear species effect was observed, with cattle calves showing higher seroconversion rates (77.12%) than buffalo calves (67.34%). Conclusions: This large-scale systematic evaluation highlights that S19 vaccine induces substantial seroconversion with regional, temporal, and species-specific differences. These insights provide crucial evidence for refining and strengthening India’s national brucellosis control program.