Spread of Yellow Fever Virus outbreak in Angola and the Democratic Republic Congo 2015-2016: a modelling study

Background: Since late 2015, an epidemic of Yellow fever virus (YFV) has caused over 6,554 suspected cases in Angola and the Democratic Republic of Congo, including 387 deaths. We sought to understand the spatial spread of this YFV outbreak to optimise the use of the limited available vaccine stock. <br/>Methods: We jointly analysed datasets describing the epidemic of YFV, vector suitability, human demography and mobility in Central Africa in order to understand and predict the expansion of YFV. We used a standard logistic model to infer the district YFV infection risk over the course of the epidemic in the region. <br/>Findings: Early spread of YFV was characterized by fast exponential growth (doubling time of 5-7 days) and fast spatial expansion (49 districts reporting cases after only three months) from Luanda, the capital of Angola. Early invasion was positively correlated with high population density (0·52, 95% CI: 0·34, 0·66). The further away locations were from Luanda the later the invasion date (0·60, 95% CI: 0·52, 0·66). Districts with higher population densities also featured higher risks of sustained transmission. A model that captured human mobility and vector suitability successfully discriminated districts with high risk of invasion from others. If at the start of the epidemic sufficient vaccines had been available to target 50 out of 313 districts in the area, our model would have correctly identified 27 (84%) of the 32 districts that were eventually affected. <br/>Interpretation: Our findings reveal the contributions of ecological and demographic factors to the ongoing spread of the YFV outbreak and provide estimates for where vaccines may be prioritised, although other constraints (e.g. vaccine supply and delivery) need to be accounted for before such insights may be translated into policy.