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This Letter should have contained an associated Creative Commons statement in the Author Information section. In addition, the Fig. 3c legend should have stated that the bar chart was adapted, with permission, from Ebola response roadmap - Situation report, Figure 3; http://www.who.int/csr/disease/ebola/situation-reports/en (accessed 1 April 2015)1. These issues have now both been corrected in the online versions of the paper.