A Single Vaccination of Nonhuman Primates with Highly Attenuated Smallpox Vaccine, LC16m8, Provides Long-term Protection against Monkeypox.

Monkeypox virus (MPXV) causes human monkeypox (human MPX), which is a similar disease to smallpox in humans. A previous study showed that a single vaccination of monkeys with LC16m8, a highly attenuated smallpox vaccine, protected them from MPX from 4-5 weeks post-vaccination. In this study, we evaluated the long-term efficacy of a single vaccination with LC16m8 in a nonhuman primate model of MPXV infection. The monkeys were inoculated with either LC16m8, Lister (parental strain of LC16m8), or a mock-up vaccine, and then challenged with MPXV via a subcutaneous route, at 6 and 12 months after vaccination, which we compared with either Lister or the mock-up vaccination. The LC16m8 monkeys exhibited almost no MPX-associated symptoms, whereas most of the naïve monkeys died. LC16m8 generated the protective memory immune response against MPXV, as suggested by the immediate viremia reduction and the response of the IgG antibody. The results demonstrated that the vaccination of monkeys with a single dose of LC16m8 provided durable protection against MPXV for longer than one year after immunization. The results suggest that the vaccination of humans with LC16m8 could induce long-term protection against MPXV infection.