While the advantages of LBVs are widely accepted, there are several key considerations associated with their successful development.
Most importantly, researchers developing an LBV must generate a vaccine strain that meets a delicate balance. The strain
must reach an appropriate level of attenuation to be safe and be sufficiently immunogenic to ensure protective efficacy. Traditionally,
live attenuated vaccines were developed by passing the pathogens under in vitro conditions until they had lost virulence for humans. This empirical approach was taken in the case of the M. bovis BCG vaccine
strain. The BCG strain was attenuated by Calmette and Guerin between 1908 and 1920 by 231 serial passages of a virulent M.
bovis strain through bile salts.4 During in vitro passage, the M. bovis microbes became attenuated because of the loss of numerous gene complexes, as was demon-strated by
a recent genome analysis of the vaccine strain.7
The second example, S. typhi Ty21a, underwent a more targeted attenuation approach. Germanier and FFCrer reasoned that a S. typhi strain, which is sensitive to galactose and could not express a polysaccharide coat (which protects the bacteria from immune
responses), should be attenuated.8 They generated the vaccine strain Ty21a in the early 1970s by chemical mutagenesis of wildtype S. typhi using nitrosoguanidine and screening for clones that had a phenotype, which is negative in the enzyme galactose epimerase,
resulting in galactose sensitivity, and which is also unable to express the Vi-polysaccharide capsule.8 As a result of the chemical mutgenesis method, the strain was also mutated in genes responsible for amino acid biosynthesis
and stress resistance, making it auxotrophic and less resistant to environmental stresses.
Finally, Kaper and Levine used genetic engineering technology in the 1980s to generate the V. cholerae vaccine strain CVD 103-HgR.6 They reasoned that the main virulence factor of V. cholerae is the expression of the cholera toxin. CVD 103-HgR was derived from a wildtype V. cholera strain by the targeted deletion of 95% of both chromosomal copies of the ctxA gene, which encodes the toxic A subunit of
the cholera toxin while keeping the expression of the nontoxic but immunogenic B subunit, leading to intermediary strain CVD
103. Subsequently, they inserted a mercury resistance marker into the genome to readily allow for identifying the vaccine
strain and its differentiation from wildtype organisms on vaccination.6
The safety and immunogenicity of vaccines are tested in animal models before they enter clinical trials. However, in the
case of LBVs this is only possible if appropriate animal models exist, which is the case only for some bacteria. We would
like to refer interested readers to a detailed article by Passetti, et al., which describes the challenges when developing
an animal model that allows meaningful vaccine testing for S. typhi-based LBVs.9
There are general differences in developing therapeutic agents for vaccines. All vaccines licensed to date have a prophylactic
effect against infectious diseases. For some infectious pathogens, clear correlates of protection are known, for example,
for threshold antibody titers. In such cases, the clinical development may be quite smooth. However, for many diseases such
correlates of protection do not exist. In such cases, vaccines are administered to healthy individuals in the pivotal Phase-3
trials, and researchers then have to wait for the trial participants to get infected by the pathogen the vaccine should protect
against and for this pathogen to cause disease. Differences in the incidence of infection or disease between the vaccine and
placebo group then allow the researchers to calculate the protective efficacy of the vaccine. For some infectious diseases
with high incidence, such a clinical trial may take just a few months. However, for diseases like tuberculosis, for which
clinical symptoms may occur only more than a decade after initial infection, Phase-3 trials may take 10–20 years. Furthermore,
the incidence of a disease may be low, requiring enormous numbers of clinical trial participants to get significant data.
Therapeutics are administered to ill people to cure them or control disease whereas vaccines are administered to healthy individuals
to protect them from becoming ill. The acceptance of side effects and safety risks is therefore much lower for vaccines in
comparison to therapeutics. Hence, vaccines have to undergo careful pre- and postlicensure safety studies.