Some
vaccines, such as
diphtheria toxoid and acellular
pertussis vaccines (aPVs), may favor the emergence of less pathogenic strains of the respective bacteria they target. This review discusses the impact of the wide use of aPV on Bordetella pertussis phenotype evolutions and their beneficial consequences in the light of the
diphtheria toxoid immunization program experience and structuring evidence review in a causal analysis following Bradford Hill's causality criteria. All aPVs contain the
pertussis toxin (PT), the main
virulence factor of B.
pertussis, alone or with one adhesin (filamentous
hemagglutinin (FHA)), two adhesins (FHA and
pertactin (PRN)) or four adhesins (FHA, PRN and two fimbriae (
Fim 2/3)). In countries where the coverage of aPVs containing PRN is high, PRN negative B.
pertussis isolates are increasing in prevalence, but isolates nonproducing the other
antigens are rarely reported. We hypothesize that the selective pressure at play with PRN should exist against all aVP
antigens, although detection biases may hinder its detection for other
antigens, especially PT. PT being responsible for clinically frank cases of the disease, the opportunity to collect PT negative isolates is far lower than to collect PRN negative isolates which have a limited clinical impact. The replacement of the current B.
pertussis by far less pathogenic isolates no longer producing the factors contained in aPVs should be expected as a consequence of the wide aPV use.