The global outbreak of the 2022 monkeypox virus infection of humans and the 2023 documentation of a more virulent monkeypox in the Democratic Republic of the Congo raised public health concerns about the threat of human-to-human transmission of zoonotic diseases. Currently available vaccines may not be sufficient to contain outbreaks of a more transmissible and pathogenic orthopoxvirus. Development of a safe, effective, and scalable vaccine against orthopoxviruses to stockpile for future emergencies is imminent. In this study, we have developed an mRNA vaccine candidate, ALAB-LNP, expressing four vaccinia viral antigens A27, L1, A33, and B5 in tandem in one molecule, and evaluated the vaccine immunogenicity in rodent models. Immunization of animals with the candidate mRNA vaccine induced a potent cellular immune response and long-lasting antigen-specific binding antibody and neutralizing antibody responses against vaccinia virus. Strikingly, the sera from the vaccine-immunized mice cross-reacted with all four homologous antigens of multiple orthopoxviruses and neutralized monkeypox virus in vitro, holding promise for this mRNA vaccine candidate to be used for protection of humans from the infection of monkeypox and other orthopoxvirus.

The global outbreak of the 2022 monkeypox virus infection of human raised the public health concerns of the threat of human-to-human transmission of zoonotic diseases. Given the evidence that other orthopoxviruses including cowpox and camelpox were also reported infectious to human, and that the reemerging risk of smallpox as a bioterrorist or accidental laboratory escape exists, there is an urgent need to develop a poxvirus vaccine with a broad protection of orthopoxviruses to stockpile for future emergency. Extensive studies of vaccinia virus (VACV) suggested that multiple VACV antigens, such as A27, L1, A33 and B5, showed high level similarity in terms of immunogenicity to their respective homologous antigens of other orthopoxviruses. These findings paved the ground for VACV antigens to be used as potential vaccine targets for development of a universal poxvirus vaccine. In this study, we construct a novel poxvirus vaccine candidate, mRNA-ALAB-LNP, encoding four vaccinia viral antigens A27, L1, A33 and B5. Strong anti-L1-specific antibody and moderate anti-A33-, anti-A27- and anti-B5-specific antibody responses were induced in mice after a single immunization. The antibody responses to all four antigens were significantly boosted after the second shot with all IgG titers >5 logs and highest being anti-A33 IgG. The high level of binding antibodies showed potent neutralizing capability against vaccinia virus. Specific IFN-γ responses were detected to all four antigens with the highest cellular response being that induced by the same antigen, A33. When evaluating the cross reactivity, equivalent or better serum IgG responses were seen in responses to corresponding monkeypox antigens A35, M1, A29 and B6, in comparison to vaccinia antigens. Apparently, the mRNA vaccine encoding four vaccinia antigens induced immunity not only to vaccinia virus but also to monkeypox, suggesting that the mRNA-ALAB may be a candidate for potential vaccine development against infection of monkeypox, smallpox and possibly other orthopoxviruses.