The strength of 70%: revision of a standard threshold of rabies control

The success of oral vaccination of foxes (ORV) conceptually is linked to the immunisation of host individuals beyond the herd immunity threshold. However, field evidence and theoretical analysis suggests that mathematically derived values of herd immunity might be rather conservative and, moreover, restrict the adjustment of standard ORV protocols in the case of limited resources. Here, the relationship between baiting effort, duration of ORV programmes and rabies elimination is analysed. An individual-based, spatially explicit model for the control of rabies in foxes that incorporates the important peculiarities of the vaccination process, i.e. the spatial distribution of infected hosts, irregular home-range use, heterogeneous bait coverage etc., is applied. Using multiple repetitions of simulated ORV programmes, the control outcome is analysed in a chance-like fashion overriding the yes-or-no prediction inherent in the herd immunity concept. It is shown why control planning must not only aim at particular immunisation levels but, simultaneously, has to specify the allowed time horizon of control success. It is demonstrated that planning a higher chance of elimination increases necessary effort non-linearly. It was found that low immunisation results (i.e. 50%) still provide a reasonable chance of control success. The potential changes in ORV planning and evaluation allowing for the integration of risk concepts in strategies are discussed.