Ebola was first identified in 1976 in southern Sudan and in what is now the DRC, near the Ebola river. Of the known species, the Zaire ebolavirus has caused most major outbreaks, and it is the one against which the single-dose Ervebo vaccine is licensed and WHO-prequalified. The current outbreak is caused by the Bundibugyo virus, a different species, and no vaccine or drug is specifically approved against it. That is why the WHO's vaccine experts have recommended a full-scale trial of Ervebo in the DRC - to establish whether it offers cross-protection - rather than simply rolling it out as a proven countermeasure. Ring vaccination, the strategy of immunising the contacts of confirmed cases and their contacts, depends on having a vaccine known to work.

Two features make this outbreak harder to contain than its scale alone suggests. First, geography: Bas-Uele is a vast, thinly connected province bordering the Central African Republic, with poor roads, limited communications and displaced populations from armed violence - exactly the conditions under which contact tracing fails. Second, the pathogen: because the strain is Bundibugyo rather than Zaire, responders cannot fall back on the vaccine-plus-ring-vaccination playbook that ended the 2018-20 epidemic in eastern Congo. For the international-affairs syllabus, the outbreak is a live case of the gap between the countermeasures that exist and the pathogens that actually circulate, and of the role of the WHO's International Coordinating Group in allocating scarce vaccine stockpiles.