The effects of different types of imperfect vaccines on population-level transmission of diseases were investigated using models that consider both homogeneous and age-specific mixing. The models were extensions of the standard SEIR equations with an additional vaccinated component that accounts for different ways vaccine protection can fail, including failure in degree (“leakiness”), take (“all-or-nothingness”) and duration (waning of vaccine-derived immunity).
Results/Conclusions
The reduction in transmission due to vaccination can be very different for age-structured models than what it would be for homogenous models. In particular, this could be much larger for an age-structured model if the contact rates between children is higher than that between adults. Analytic expressions for age-specific “vaccine impacts” were derived. The overall vaccine impact was shown to be bounded between the impact for the oldest age class and that of the youngest age class. These can result in different conclusions when fitting homogeneous and age-structured models to the disease data.