The vaccination of a range of age cohorts on a periodic basis – termed pulse vaccination – for the control of common childhood infections, may, under some circumstances, be an attractive alternative or addition to routine infant immunization schedules (Quadros et al. 1991, Agur et al. 1993). Following from the recent work of Agur et al. (1993), studies are in progress in an attempt to gain a better understanding of the mechanism by which pulse vaccination influences the infection dynamics of directly transmitted ‘close contact’ infections and to explore the merits of such immunization programmes when compared with routine continuous cohort immunization. The preliminary work presented here outlines a conceptual framework for understanding how pulse vaccination programmes effect control on infection transmission, and for establishing rough criteria for the necessary interval between pulses, Tv, for a defined age range vaccinated, and analyses some of the temporal changes induced by pulse strategies.

The basic criterion for control of an infection transmitted by close contact, such as measles, is to keep susceptible numbers, X, below that required to establish an epidemic, i.e. the equilibrium or threshold number of susceptibles, XT (or proportion x* = 1/R0). Assume a closed homogeneously mixing stationary population, exhibiting Type I survival (step function mortality), with an infection at stable endemic equilibrium. Further assume that all individuals are born susceptible, remaining so until of age A, the average age at infection, whence they become infected (for a negligible period of time), and rapidly move into the immune class in which they remain until death at age L (Anderson and May 1991).