The population dynamics of disease transmission may interact with
the kinetics of immunity to further generate counter-intuitive phenomena that
may affect the potential impact of control programs. The introduction of
pertussis vaccine has been implicated in the recent increase in whooping cough
rates in adults. Analysis of
age-specific whooping cough rates in Massachusetts showed an increase in
disease incidence in adults that was far greater than would be expected due to
the reduction in the force of infection alone Lavine 2011. Natural immunity to
pertussis wanes over one’s lifetime, though the duration of protective immunity
following infection is a matter of dispute. The introduction of pertussis
vaccination reduced the rate at which individuals experience their first
infection as well as the rate at which immunized individuals were re-exposed to
natural infection in the environment.
Natural exposure of previously immune individuals results in a boosting
of immunity, analogous to a booster vaccine, that results in no disease but
conveys additional protection. The
lowered prevalence of pertussis resulting from vaccination meant that many
individuals lost their protective immunity prior to re-exposure and thus were
not protected upon subsequent natural exposure.
The resulting increase in adult cases arises from the combined
time-scales of waning immunity and subsequent natural encounter with
pertussis.