Design Of Immunization Programmes Research Articles

Epidemiology of varicella zoster virus infection in Canada and the United Kingdom.

Many countries are currently studying the possibility of mass vaccination against varicella. The objective of this study was to provide a comprehensive picture of the pre-vaccine epidemiology of the varicella zoster virus (VZV) to aid in the design of immunization programs and to adequately measure the impact of vaccination. Population-based data including physician visit claims, sentinel surveillance and hospitalization data from Canada and the United Kingdom were analysed. The key epidemiological characteristics of varicella and zoster (age specific consultation rates, seasonality, force of infection, hospitalization rates and inpatient days) were compared. Results show that the overall epidemiology of varicella and zoster is remarkably similar between the two countries. The major difference being that, contrary to Canada, the epidemiology of varicella seems to be changing in the United Kingdom with an important decrease in the average age at infection that coincides with a significant increase in children attending preschool. Furthermore, differences exist in the seasonality between the United Kingdom and Canada, which seem to be primarily due to the school calendar. These results illustrate that school and preschool contact patterns play an important role in the dynamics of varicella. Finally, our results provide baseline estimates of varicella and zoster incidence and morbidity for VZV vaccine effectiveness and cost-effectiveness studies.

Spatial heterogeneity and the design of immunization programs

Conventional epidemiological models usually assume homogeneous mixing, with susceptible and infected individuals mingling like the molecules in an ideal gas. It has recently been noted, however, that variability in transmission rates—arising, for example, from some hosts being in dense aggregates while others are in small or remote groups—can result in the intrinsic reproductive rate, R 0, of a microparasitic infection being greater than would be estimated under the usual assumption of homogeneous mixing; this implies the infection may be harder to eradicate under a homogeneously applied immunization programme (that is, a larger proportion of the population must be vaccinated) than simple estimates might suggest. In this paper we consider a spatially heterogeneous population arbitrarily subdivided into n groups, with one transmission rate among individuals within any one group, and another, lower transmission rate between groups. We define an optimum eradication program as that which—treating different groups differently—achieves its aim by immunizing the smallest overall number in each cohort of newborns, and we show this optimum program requires fewer immunizations than would be estimated under the (false) assumption that the population is homogeneously mixed. We prove this result in general form, and illustrate it for some special examples (in particular, for a population subdivided into one large ”city“ and several small ”villages“).

Spatial, temporal, and genetic heterogeneity in host populations and the design of immunization programmes.

The relevance of heterogeneity in infectious disease transmission to the design of immunization programmes is examined by the use of simple deterministic models of recurrent epidemic behaviour. In the context of spatial heterogeneity it is shown that an optimal vaccination programme, which concentrates on groups with relatively high density, can lead to eradication with overall coverage levels less than those estimated by a model which assumes homogeneity in host spatial distribution. Age-related heterogeneity in transmission can result in higher or lower coverage levels than those predicted by models which assume age-independent transmission, depending on the quantitative detail of how transmission changes with age. Genetic heterogeneity in host susceptibility to infection is shown to produce effects that are similar to those induced by age-related changes in transmission. It is concluded that much greater attention should be focused on transmission heterogeneity in future work on models of recurrent epidemic behaviour and the design of immunization programmes.