Pertussis vaccine and transmission
Pertussis is considered an endemic disease, characterized by an epidemic every 2–5 years. This rate of exacerbations has not changed, even after the introduction of mass vaccination – a fact that indicates the efficacy of the vaccine in preventing the disease but not the transmission of the causative agent (B. pertussis) within the population .
The effects of whole-cell pertussis vaccine wane after 5 to 10 years, and infection in a vaccinated person causes nonspecific symptoms (3-7). Vaccinated adolescents and adults may serve as reservoirs for silent infection and become potential transmitters to unprotected infants (3-11). The whole-cell vaccine for pertussis is protective only against clinical disease, not against infection (15-17). Therefore, even young, recently vaccinated children may serve as reservoirs and potential transmitters of infection.
Case definition has been particularly problematic in all of the recent DTaP vaccine efficacy trials. For uniform comparative purposes a case definition was suggested by a WHO expert committee.39 This definition required 21 days of paroxysmal cough plus laboratory confirmation of pertussis in the subject or household contact. There are 2 problems with this definition. The first is that a substantial number of B pertussis infections in unvaccinated children are mild and would not meet the case definition. The second is that all pertussis vaccines tend to modify duration and severity of disease rather than completely preventing illness.18,2326-29,35,36,40 Therefore, the WHO definition has made all vaccines look better than they are and it has tended to obscure differences between vaccines.
Most of the studies on the pertussis vaccine and transmission have been about the whole-cell vaccine, but it has generally been considered to be more effective than the new acellular one. To learn more about why the acellular vaccine does not prevent transmission, try this Google Scholar search.
For example, this study concludes:
In summary, booster immunization of adults with acellular pertussis vaccines was not found to increase bactericidal activity over preimmunization levels. Identifying ways to promote bactericidal immune responses might improve the efficacy of acellular pertussis vaccines.
Also, notice the small print of an advertisement for Adacel, a DTaP booster:
“It is unknown whether immunizing adolescents and adults against pertussis will reduce the risk of transmission to infants.”
Also, from a WHO meeting on the acellular vaccine:
In discussion, Dr Cherry pointed out that in Japan also the reported pertussis incidence in children under three months of age has not declined substantially with return to a high vaccination coverage.