Assessment of two-dose vaccination schedules: availability for vaccination and catch-up strategies

An age-specific transmission model is used to discuss aspects of two-dose vaccination schedules. Motivated by measles, the model allows for a period of maternally acquired immunity and assumes that infection leads to permanent immunity. The model expresses the coverage at the second dose in terms of the availability of individuals for vaccination, where availability depends on whether the individual received the first vaccination and the time between the first and second doses. It is emphasized by illustration that medium-term performance is a more appropriate assessment of vaccination schedules than eventual outcome, particularly since elimination of measles does not seem possible at this time. Failure to take account of individuals' availability for vaccination when designing vaccination schedules can lead to a poor control strategy and to unjustified optimism. When disease transmission is in equilibrium, an instantaneous change to a vaccination schedule with high coverage often results in a period of low incidence followed by a series of substantial epidemics occurring over a long period of time. A gradual increase to the same level of vaccination can dampen these epidemic waves significantly and gives similar numbers of cases in the medium and long terms. It is found that a strategy based on doses offered at ages 1 and 11 years is far from optimal, but the inclusion of a catch-up vaccination at school entry, offered only to previously unvaccinated individuals, can improve this two-dose strategy dramatically.