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Modelling the decay of post-vaccination immunity rate in an animal population: Peste des petits ruminants in western Africa

Hammami P., Lancelot R., Lesnoff M.. 2016. In : 1st Joint International Conference AITVM - STVM. Giessen : Verlag, p. 42-42. Joint International Conference AITVM - STVM. 1, 2016-09-04/2016-09-08, Berlin (Allemagne).

Peste des petits ruminants (PPR) is the most serious infectious viral disease affecting small ruminants in Africa, the Middle East and Asia. A global PPR control strategy based on mass vaccination in countries where PPR is endemic was recently designed by international organizations. Sahelian Africa is one of the most challenging regions for PPR control. In such a region, strong seasonality in mating and offtake (slaughtering associated with Tabaski, a Muslim festival whose date is changing according to the lunar calendar) results in a complex seasonal and interannual population dynamics which might be important to consider for the implementation of vaccination campaigns. The post-vaccinal population immunity rate (PIR) persistence is poorly understood. Based on a previous study, we developed a predictive model of PIR dynamics built on a seasonal matrix population model and accounting for post-infectious and colostral immunity. Calibrated with demographic and epidemiological parameters estimated from a long-term follow-up survey and literature review, the model was used to simulate the PIR dynamics following a vaccination campaign against PPR in a Sahelian sheep population and to define the best period to implement vaccination campaigns. Simulations showed Tabaski date did not influence the PIR. Decreasing vaccination coverage from 100% to 80% had limited effects. Though lower vaccination coverage did not provide a good immunity rate (PIR < 70%). Considering two successive vaccination campaigns - like proposed in the Global strategy, might result in higher PIR. According to the model, the best vaccination period would range from May to July. This period is not adequate for practical implementation of vaccination campaigns in the Sahel. A trade-off between PIR and operational features could be to implement vaccination campaigns in September-October. This model is a first step towards better decision support for animal health authorities. It might be adapted to other species, farming systems or diseases.

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