Enhanced mouse virulence of mpox virus clade Ib over clade IIb despite genomic changes caused by human-to-human transmission

The recently emerged clade Ib of mpox virus (MPXV) is spreading rapidly across Central and West Africa raising concerns about its potential virulence. Similar to clade IIb lineage B.1, which was responsible for the 2022 global outbreak, clade Ib exhibits sustained human-to-human transmission and a pattern of APOBEC3 associated genomic mutations. Here, we show that clade Ib displays enhanced cell-to-cell dissemination in vitro compared to clade IIb. Additionally, using the CAST mouse model, we show that clade Ib retains a higher level of virulence than that of the markedly attenuated clade IIb. Clade Ib leads to significant weight loss and high mortality in animals following both intraperitoneal and intranasal challenge. Histopathological analysis revealed more severe and extensive lung lesions in clade Ib infected animals, accompanied by a broader distribution of viral antigens. Moreover, clade Ib, unlike IIb, disseminated efficiently to internal organs. These findings indicate that clade Ib MPXV has not undergone attenuation after human-to-human transmission to the extent observed in clade IIb and underscore the need for surveillance and preparedness against new emerging MPXV lineages.