Post-vaccination sero-monitoring of bovine calves in Indian subcontinent: A review on progress towards brucellosis control.

Brucellosis caused by Brucella abortus remains a major zoonotic challenge in India, causing substantial economic losses in livestock and public health risks. Although India has implemented nationwide S19 vaccination in cattle and buffaloes, comprehensive evaluation of its effectiveness across different regions and between species has been lacking. This study sought to assess post-vaccination sero-conversion rates in calves of aged 4-8 months across five Indian states/union territories (UTs), examine species-specific differences in vaccine response between cattle and buffaloes, and generate practical recommendations to optimize the national brucellosis control program. A multi-phase sero-monitoring study analyzed 19,893 serum samples, comprising 16,085 cattle calves and 3609 buffalo calves during 2021-2024. The samples were collected from Andhra Pradesh, Chandigarh, Haryana, Odisha, and Tamil Nadu and tested using laboratory standardized indirect enzyme-linked immunosorbent assay (iELISA).Significant disparities were observed between species with overall sero-conversion rates of 75.87 % in cattle versus 67.22 % in buffaloes (p < 0.001). Tamil Nadu demonstrated exceptional performance (84.61 %; 95 % CI: 84.20-84.95), with districts like Namakkal achieving 100 % sero-conversion. Other regions showed varied efficacy: Chandigarh (80.77 %), Andhra Pradesh (69.52 %), and Haryana (69.43 %) consistently exhibited 10-15 % lower rates in buffaloes (p < 0.001). Odisha displayed notable phase-wise improvement (71.84 %; CI: 70.02-71.12), with Jagatsinghpur district reaching 97.85 % and buffalo calves improving from 17.64 % (Phase I) to 57.5 %. While, the S19 program achieves moderate efficacy but highlights species-specific disparities and replicable success models such as Tamil Nadu. Targeted buffalo vaccination strategies and adoption of best practices are recommended to achieve > 80 % vaccination coverage thereby herd immunity. Overall, this study provides an evidence-based framework for strengthening India's brucellosis control program and contributing to global eradication efforts.