The Ebola outbreak in the Democratic Republic of the Congo has become the second-largest on record. A reasonable question follows: there is an Ebola vaccine, and it worked in previous epidemics — so why is this one still growing?

The answer turns on a detail that rarely survives the headline. Ebola is not one virus.

Several viruses, one name

“Ebola” refers to a group of related viruses, several of which cause disease in people. They are named after the places where they were first identified: Zaire, Sudan, Bundibugyo, Taï Forest.

They cause broadly similar illness, but they are genuinely different viruses — different enough that protection against one does not reliably transfer to another. The current outbreak is Bundibugyo, first identified in a district of Uganda in 2007.

Why the vaccines target one species

The licensed vaccines were developed against the Zaire species, and for sound reasons: it had caused the largest and deadliest outbreaks, including the West African epidemic of 2014–16 that killed more than 11,000 people. That epidemic generated the funding, the political will and — grimly — the case numbers needed to test a vaccine at scale.

A vaccine works by teaching the immune system to recognise a specific structure on the surface of a virus. Where a related virus presents a sufficiently different structure, the immune system may not recognise it, or may respond too weakly to matter.

So the honest position on Bundibugyo is not that vaccination is useless. It is that the evidence supporting it is thin, because the trials were not done against this species — and thin evidence is a poor basis for a mass campaign in the middle of an emergency.

Why the trials were not done

This is where the science meets economics.

Testing a vaccine requires an outbreak large enough to measure whether it works. Outbreaks of the less common species have historically been smaller and have ended before trials could be organised. That is a good outcome for the people involved and it leaves a gap in the evidence.

There is also no commercial market. Vaccines against diseases that appear intermittently in low-income regions do not repay development costs. Where such products exist at all, they exist because governments, foundations or international bodies paid for them — and that funding follows attention, which follows large outbreaks.

The result is a system that develops the countermeasure after the catastrophe rather than before it.

What is used instead

Where a proven vaccine is unavailable, response falls back on methods that predate vaccines entirely — and that still work.

Find every case. Trace everyone they had contact with. Monitor those contacts for symptoms and isolate them promptly if symptoms appear. Ensure safe burials. Protect health workers with equipment and training.

None of that requires a laboratory breakthrough. All of it requires people, transport, fuel, equipment and — above all — communities willing to cooperate. It is labour-intensive, unglamorous, and the first thing to break down when cases outpace the staff available.

Uganda’s own outbreak was declared over on 28 July after 42 days without a new locally transmitted case, which is the standard interval — twice the maximum incubation period. That demonstrates containment remains achievable without a species-matched vaccine.

What could change

Two things would alter this picture.

The first is vaccines designed against multiple species at once, which are under development. The second is a different funding model — paying to have countermeasures ready for diseases that have not yet caused a crisis, rather than after they have.

Both are well understood in principle. Neither is quick, and neither helps the current outbreak.

The point worth keeping

When a disease returns despite a vaccine existing, the reason is rarely that the science failed. More often it is that the science was funded for one version of the problem and not another.

That is a decision, not an accident — and decisions can be made differently.

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Background: World Health Organization guidance on Ebola virus species and vaccines; outbreak reporting, August 2026.