Monkeypox virus clade IIb isolate exhibits reduced virulence relative to clade IIa isolates in multiple murine models.

Monkeypox virus (MPXV) is the causative agent of mpox disease in humans. The virus comprises two clades, Central African clade I and West African clade II, with case fatality rates of ~11 and ~4%, respectively. Since the discovery of mpox disease in 1970, the virus has been restricted to Africa. However, in 2022, a previously unrecognized subclade IIb caused the largest global outbreak of mpox disease with a case fatality rate of ~0.2%. The difference in virulence of MPXV subclades in human infection warrants further investigation; however, one critical limitation is the lack of susceptible small animal models. In this study, we investigated the susceptibility of four murine models, including CAST-EiJ and three immunocompromised models (C57BL/6 IMPORTANCE: Mpox is an emerging human disease caused by four distinct MPXV subclades (Ia, Ib, IIa, and IIb). Despite genetic similarities, the case fatality rate varies considerably between the subclades: Ia (~11%), Ib and IIa (~4%), and IIb (~0.2%). Since 2022, multiple mpox outbreaks have occurred due to previously unrecognized subclades, leading to the declaration of two public health emergencies by the World Health Organization. This unprecedented global spread, coupled with the variation in the severity of human disease, underscores the importance of research into the pathogenesis of emerging MPXV subclades. However, a critical limitation is the lack of suitable small animal models. This study identifies three additional murine models susceptible to MPXV clade II infection and demonstrates significant virulence differences between clades IIa and IIb. These models will enable a rapid characterization of previously unrecognized subclades and will facilitate countermeasure development.