Dynamic Feedbacks: Interventions themselves cause changes in transmission dynamics that
may lead to otherwise counter-intuitive outcomes. Dynamic models provide a tool
to explore these feedbacks a priori
to anticipate shifting epidemiology. The non-linear dynamics of many infectious
disease systems mean that the phenomena in future, unexplored regimes of
parameter space, such as following the introduction or increase of vaccination,
are unlikely to be a simple extrapolation of currently observed phenomena.
Anticipating these phenomena can prevent concerns about the performance of
control efforts. Introduction of
vaccination is expected to result in a numerical increase in cases in older age
classes because fewer will be immunized by natural infection as children Ferrari 2013 METCALF 2012 . Vaccination can shift the
period of outbreaks Earn 2000; this may result in outbreaks becoming more frequent
(i.e. a switch from multi-annual to annual cycles) or less frequent. The latter can have the counter-intuitive
result of increasing the case burden in outbreak years relative to pre-vaccine
levels, even while reducing the average annual burden Pitzer 2012.