I177L-based five-gene deletion attenuates a genotype I/II recombinant ASFV but fails to provide complete protection against parental or heterologous strains

African swine fever (ASF), caused by the African swine fever virus (ASFV), is a highly contagious and often fatal disease that poses a serious threat to the global pig industry. The emergence of genotype I/II recombinant ASFV strains has further complicated vaccine development and disease control. This study reveals that ASFV-GZΔI177LΔCD2vΔMGF, an effective candidate vaccine strain against genotype II ASFV, fails to provide protection against lethal challenge from the genotype I/II recombinant strain ASFV-HN. Although a five-gene-deleted strain based on ASFV-HN with I177L deletion (ASFV-HNΔI177LΔCD2vΔMGF) exhibits significantly reduced pathogenicity in pigs and was observed to delay mortality in pigs upon lethal challenge, it did not effectively protect against lethal challenge from either the parental strain or genotype II virulent strains. These findings reveal the unique virulence characteristics of genotype I/II recombinant ASFV strains and highlight the limitations of current vaccine strategies. They also emphasize the need to develop new approaches to address the risks posed by recombinant ASFV strains and to account for the importance of viral genotype and recombination events.