An inactivated trivalent virion-based vaccine protects against aerosol challenge with encephalitic alphaviruses in mice and macaques

Venezuelan (VEEV), Eastern (EEEV), and Western (WEEV) equine encephalitis viruses are mosquito-transmitted alphaviruses in the family Togaviridae with the potential to cause fatal neuroinvasive disease in humans. These viruses can also be infectious when aerosolized and, thus, are potential biothreat agents. Human infection can progress rapidly to encephalitis, with fatality rates of 1 to 10% in symptomatic VEEV and WEEV infections and 30 to 70% in symptomatic EEEV infections. Currently, there are no antiviral agents or vaccines approved for encephalitic alphaviruses. An investigational live-attenuated VEEV vaccine was generated more than 40 years ago but is highly reactogenic, poorly immunogenic, and causes disease in up to 20% of recipients. Formalin-inactivated vaccines for EEEV and WEEV are also poorly immunogenic and no longer available. Here, we developed a trivalent vaccine against VEEV, EEEV, and WEEV using a combination of attenuated chimeric Sindbis-VEEV/EEEV/WEEV viruses to streamline production and an H₂O₂ inactivation treatment for enhanced safety and virion surface epitope preservation. The vaccines were adjuvanted with alum and tested for immunogenicity and protection in mouse and nonhuman primate models of lethal, aerosolized alphavirus infection. Two doses conferred complete protection in mice, and a similar regimen showed substantial or complete protection in nonhuman primates when administered at low and high doses, respectively. These results suggest that this inactivated vaccine is effective against the three encephalitogenic alphaviruses and may meet the need to counter the public health threat and biothreat posed by these viruses.