Eastern equine encephalitis virus-vaccinated mice are protected against Madariaga virus despite negligible neutralizing antibody titers

Madariaga virus (MADV) is an understudied member of the eastern equine encephalitis virus (EEEV) complex that circulates widely in Latin America and may be geographically expanding. Spillover infections can cause severe disease in humans and equids. Despite these public and animal health concerns, no MADV vaccines or treatments are available. In this context, commercially available equine vaccines against the closely related North American EEEV (NA EEEV) could potentially provide heterologous protection against MADV. However, previous studies have shown weak or undetectable MADV-neutralizing antibody responses in equids and humans immunized with inactivated NA EEEV vaccines, and whether protection can occur despite these limited neutralizing antibody responses remains unknown. We evaluated a commercial equine trivalent inactivated NA EEEV vaccine against MADV challenge in NIH Swiss mice. Animals received two vaccine doses 14 days apart and were challenged 14 days later. Vaccination provided protection against lethal disease, whereas 36% mortality occurred in the unvaccinated group. Overall, vaccinated animals developed fewer and less severe clinical signs. MADV RNAemia was not detected following vaccination, and tissue dissemination was significantly reduced compared with the unvaccinated group. Central nervous system lesions were observed only in the unvaccinated group. Despite protection, most vaccinated mice had no detectable MADV-neutralizing antibody titers, and those that seroconverted developed only low titers, whereas most unvaccinated mice developed intermediate to high titers. These findings demonstrate that an inactivated NA EEEV-containing vaccine can protect against severe MADV disease despite limited MADV-neutralizing antibody responses.