The deletion of the EP402R and MGF505/360 genes attenuates a genotype I/II recombinant ASFV but fails to confer complete protection against homologous or genotype II challenge in pigs.

African swine fever (ASF), a pig disease caused by ASFV, is highly contagious and often lethal. The recent emergence of novel ASFV with I/II genomic recombination has posed significant challenges to global ASF prevention and control. In this study, we used the Chinese I/II genomic recombinant virulent strain ASFV-HN10005 (ASFV-HN) to construct two gene-deleted viruses. ASFV-HNΔMGF lacks the MGF505-1R-MGF360-14L genes cluster (including MGF505-1R, -2R, -3R, and MGF360-12L, -13L, -14L), and ASFV-HNΔCD2vΔMGF lacks both EP402R and the MGF505-1R-MGF360-14L genes cluster. ASFV-HNΔMGF still showed pathogenicity in domestic pigs, while ASFV-HNΔCD2vΔMGF showed markedly attenuated virulence, with all inoculated pigs surviving. However, these pigs did not gain complete protection against subsequent lethal challenge from the parental ASFV-HN strain or the virulent II-type strain ASFV-GZ. This implies that I/II genomic recombinant ASFV may have unique biological traits and immune evasion mechanisms. Our findings indicate the need to reassess current ASFV control strategies and provide a basis for future vaccine target selection.