WANING IMMUNITY AND ITS EFFECTS ON VACCINATION SCHEDULES

A relatively comprehensive age-specific transmission model is used to determine the effect of various factors on the optimal vaccination age s in one-dose and two-dose vaccination schedules. Motivated by the sit uation for measles, the model allows the duration of immunity of newbo rns to depend on the level of immunity of the mother at the time of th e birth and allows for waning immunity as well as boosting of immunity by exposure to the disease. It is found that a significant amount of waning of disease-acquired immunity is plausible when boosting occurs but this is not an important factor in determining optimal vaccination schedules. On the other hand, plausible rates of loss of vaccine-indu ced immunity can have a substantial effect on the optimal vaccination schedule, particularly when there is no boosting of immunity. For two- dose schedules the optimal vaccination ages depend significantly on th e level of vaccination coverage achieved. In the presence of plausible rates of loss of vaccine-induced immunity for measles, it is found th at the vaccination coverage required to eradicate the disease is subst antially higher than previously suggested.