Provided are a recombinant classical swine fever virus comprising at least one substitution within the epitope of the E2 protein specifically recognized by the 6B8 monoclonal antibody, an immunogenic composition comprising said CSFV, the use of said immunogenic composition for preventing and/or treating diseases associated with CSFV in an animal, a method or a kit for differentiating animals infected with CSFV from animals vaccinated with said immunogenic composition, and an attenuated classical swine fever viruses.

Replicons of pestiviruses, in particular replicons of swine fever virus, are engineered to have a defective replication thereby having lost infectivity, and further containing a foreign gene. A replicon contains all the genetic information required for its replication, but lacks essential codons or all codons of at least one of the genes encoding the viral structural proteins E1, E2, E.sup.ms or C protein, and consequently cannot generate infectious virus particles. Particular replicons are generated with a mutated gene encoding a modified N.sup.pro protein that no longer controls the cell interferon-induction pathway. Another particular replicon lacks the genes encoding all the structural proteins, the p7 protein and the NS2 protein, and has cytopathogenic properties in transduced cells. The replicons provide a new vector system that can be used for vaccination, gene delivery and gene therapy applications in mammals, including humans, as naked RNA or packaged into any form of delivery vehicle.