Yellow fever outbreak potential in Djibouti, Somalia and Yemen: a mathematical modelling study

The importation of arbovirus diseases into new countries is a global concern. This risk is exacerbated by human movement and climate change effects. In the World Health Organization (WHO) Eastern Mediterranean regional office, three countries - Djibouti, Somalia, and Yemen - are currently classified as potential or moderate risk for yellow fever (YF) outbreaks.

Here we present a quantitative assessment of the risk of introduction and propagation of yellow fever virus (YFV) transmission in Djibouti, Somalia and Yemen. This modelling has two components: i) projecting the risk of importation of infectious individuals into the countries of interest using a radiation model of human movement and ii) estimating the risk of onward transmission given importation using a dynamic compartmental model of yellow fever virus transmission. Both components are multiplicatively combined to give an overall relative outbreak risk combining both risk of importation and risk of an outbreak given importation.

We show that areas such as the western coast of Yemen, regions of Somalia bordering Ethiopia and Kenya, and Djibouti City have potential for YF outbreaks (where the estimated probability of an outbreak given an imported infectious case is over 50%). This is due to environmental suitability for transmission based on factors such as temperature and projected human mobility between endemic and at-risk regions.

Countries bordering existing YF endemic regions are potentially vulnerable to both introduction of YF cases and subsequent outbreak spread. This promotes the awareness of YF importation potential when conducting clinical surveillance in at-risk countries.