A Simulation Model for the Control of Helminth Diseases by Chemotherapy

If an individual is successfully vaccinated he or she is protected against the corresponding infection during the duration of the induced immunity. The vaccination of a certain proportion in a population is of obvious benefit to those covered. But there is also some effect to the rest of the community: the risk of acquiring an infection is reduced. For certain kinds of infections this also results in a reduction of the incidence of disease. Some infections can only lead to disease in higher age groups, like rubella which may cause congenital malformations if the mother gets infected during early pregnancy. In this case a reduction in infection risk results in an increase of age at infection. This may lead to an increase of the total incidence of disease in the population as has been pointed out by Knox (1980), Dietz (1981) and Anderson & May (1983). This example shows that an evaluation of a vaccination strategy has to take into account the consequences on the disease incidence in the total population.