The 2003 monkeypox virus (MPXV) outbreak and subsequent laboratory studies demonstrated that the black-tailed prairie dog is susceptible to MPXV infection and that the ensuing rash illness is similar to human systemic orthopoxvirus (OPXV) infection, including a 7-to 9-day incubation period and, likely, in some cases a respiratory route of infection; these features distinguish this model from others. The need for safe and efficacious vaccines for OPVX in areas where it is endemic or epidemic is important to protect an increasingly OPXV-naïve population. In this study, we tested current and investigational smallpox vaccines for safety, induction of anti-OPXV antibodies, and protection against mortality and morbidity in two MPXV challenges. Although smallpox has been eradicated (5), orthopoxviruses (OPXVs), such as variola virus (VARV) (6, 32), monkeypox virus (MPXV) (3, 10, 57), vaccinia virus (VACV) (4, 76), and others (53,68,77), are a continuing public health concern (61, 67). Due to cellular and humoral protective effects (7,15,16,46,73), the traditional live VACV-based smallpox vaccines provide cross-protection against multiple OPXV threats (48). Unfortunately, the cessation of routine vaccination (37) has left much of the global population fully susceptible to OPXV infections (34), and the adverse effects of first-and secondgeneration vaccines (78) coupled with increases in the global immunocompromised population (59) underline the need for continued development and testing of safer smallpox vaccines (30).Testing of smallpox vaccines requires the development of relevant animal models. Current smallpox vaccine testing animal models include ectromelia virus (ECTV) infection of mice (75), VACV infection of mice (63), rabbitpox virus (RPXV) and VACV infection of rabbits (1, 18), VARV and MPXV infection of nonhuman primates (NHP) (13,19,21,25,26,31), and others (65, 74). Limitations of current models include abbreviated disease incubation periods and/or differences from human systemic orthopoxvirus disease presentation and progression The validation of alternative animal models for smallpox vaccine testing is an ongoing effort (2, 19, 39).The MPXV outbreak in the United States in 2003 (3) identified a potential alternative model as it established that MPXV can be transmitted to black-tailed prairie dogs (Cynomys ludovicianus) (42). Investigations of the MPXV outbreak and epidemiological, immunological, and pathological studies of prairie dogs infected with MPXV (8,12,27,28,35,38,43,57,58,64,69,74,80) determined that this rodent species can manifest MPXV infection similar to human systemic OPXV disease. This disease model shares a respiratory route of infection, similar disease incubation period, and similar rash illness (20,29,38,42,80) with human MPXV and VARV infections, thus making it a potentially useful small animal model for testing vaccines and therapeutics (29,70). A particular strength of this model is its 7-to 9-day incubation period, which is more similar to that seen in human disease than many other a...