The subsp. holarctica live attenuated vaccine strain (LVS) is currently the only effective vaccine against tularemia. The vaccine is approved in the U.S. only for use in clinical trials, and its future availability is undetermined because the protective response offered by the vaccine has not been well characterized, the basis of its attenuation is unknown, and LVS is fully virulent when delivered intraperitoneally in mice. The fact that the LVS provides protective immunity suggests that efforts to create a defined attenuated mutant may be fruitful, however such strains may not be available for some time.
RC503 has been historically used as a virulent Type B murine challenge strain by Russian scientists. The sequence data from this genome will serve as a comparative reference to aid in the identification of potentially attenuating mutations in the LVS and will provide further data toward identifying the genetic determinants that distinguish Type B pathogenicity and geographic distribution.