The use of live attenuated vaccines raises the important issue of their biosafety, especially if they are destined for wide distribution. It is important to demonstrate that the genetic attenuation is stable, and that reversion to virulence does not occur. Since no natural horizontal gene transfer mechanism exists between B. pertussis cells, reversion of large deletions, such as those for the dermonecrotic and ampG genes, is nearly impossible. PT was inactivated in BPZE1 by two independent mutations affecting two amino acid residues that are both critical for its ADP-ribosyltransferase activity, one being essential for substrate binding and the other for catalysis. Reversion of both to active amino acid residues is required to regain enzyme activity, which again is virtually impossible. The genetic stability of BPZE1 has been demonstrated upon continuous serial passages both in vitro and in vivo in mice for up to one year (Feunou et al., 2008). Further preclinical safety data also indicates that BPZE1 does not exacerbate airway pathology associated with allergen sensitization, although virulent B. pertussis infection exacerbates inflammation in such model (Kavanagh et al., 2010). Instead, BPZE1 administration actually protects against experimentally induced allergic pulmonary pathology, and reduces allergen-driven IL-4, IL-5 and IL-13 production (Kavanagh et al., 2010).  Finally, whereas the virulent parent strain kills MyD88-deficient mice within one to two weeks after nasal infection with 10^4 colony-forming units, all mice survive for at least one month after infection with even a 100-fold higher dose of BPZE1, and no significant weight loss is observed upon infection with BPZE1.