Post-vaccination seroconversion and lineage-level molecular characterization of classical swine fever virus in five northeastern states of India.

Classical swine fever (CSF) is a highly contagious viral disease of pigs with major economic consequences, necessitating sustained vaccination and surveillance under India’s national control programme. The present study evaluated post-vaccination seroconversion and conducted lineage-level molecular characterization of circulating classical swine fever virus (CSFV) in five northeastern states of India. An Erns-based indirect ELISA was used to assess antibody responses in pig sera collected before and 30 days after vaccination. A marked increase in seropositivity was observed following immunization, indicating successful seroconversion under field conditions. To confirm the ELISA findings, a stratified random subset of 189 post-vaccination serum samples was tested using the virus neutralization test (VNT), which demonstrated the presence of protective neutralizing antibody titers in the majority of samples. In parallel, limited molecular analysis was performed to identify circulating CSFV lineages using partial 5′ untranslated region sequences from representative field samples. Phylogenetic analysis showed clustering of field isolates within CSFV sub-genotype 1.1, distinct from the Hog Cholera Lapinized Virus vaccine strain. Overall, the integration of large-scale serological monitoring with confirmatory neutralization testing provides robust evidence of vaccine-induced immunity, while the molecular findings highlight the continued circulation of endemic CSFV lineages. These results support ongoing vaccination efforts and emphasize the value of combining immunological surveillance with targeted molecular monitoring for CSF control in India.