The realities of biodefense vaccines against Rickettsia.

Rickettsia prowazekii, R. rickettsii, R. conorii, and R. typhi are serious biologic weapon threats because of high infectivity of low dose aerosols, stable small particle aerosol infectivity, virulence causing severe disease, difficulty in establishing a timely diagnosis, ineffectiveness of usual empiric treatments, potential for engineered complete antimicrobial resistance, lower level of immunity, availability of the agents in nature, and feasibility of propagation, stabilization, and dispersal. Infection induces long-term immunity, killed rickettsial vaccines stimulate incomplete protection, and a live attenuated mutant stimulates strong immunity but reverts to virulence. Prospects for rational development of a safe, effective live attenuated vaccine are excellent.