The Democratic Republic of the Congo is fighting what the World Health Organization calls the second-largest Ebola epidemic on record. There is a licensed Ebola vaccine, and roughly half a million doses are available.
On 1 September, WHO published guidance saying that vaccine should be used only inside research protocols — not deployed as routine protection.
The reason is a distinction most coverage leaves out. This outbreak is not the Ebola the vaccine was built for.
The numbers
WHO’s update of 26 August 2026 recorded, for the DRC:
- 5,794 confirmed cases
- 2,786 deaths — a case fatality rate of 48.1 per cent
- 1,293 recoveries
- 60 health zones across six provinces affected
Ituri province is worst affected, with 4,802 cases across 28 of its 36 health zones. Twenty cases have been exported to Uganda and one to France.
WHO declared a Public Health Emergency of International Concern on 18 August 2026.
A case fatality rate near 48 per cent means roughly one in two diagnosed people die. For comparison, the 2014–16 West African epidemic — the largest on record — killed more than 11,000 people over two years. This outbreak reached nearly 2,800 deaths within about four months of detection in May 2026.
WHO Director-General Tedros Adhanom Ghebreyesus said it is “moving faster than any previous Ebola outbreak.”
The distinction that governs everything
“Ebola” is not one virus. It is a group of related species, and they are not interchangeable for medical purposes.
Ervebo, Merck’s licensed vaccine, was developed against Zaire ebolavirus. That is the species responsible for most large historical outbreaks, including West Africa in 2014–16, and Ervebo performed well against it — one of the genuine triumphs of recent vaccine development.
This outbreak is caused by Bundibugyo virus, a different species.
A vaccine works by training the immune system to recognise specific viral proteins. Bundibugyo’s surface proteins differ from Zaire’s. Some cross-protection is biologically plausible — the species are related — but plausible is not demonstrated. There is no human efficacy data for Ervebo against Bundibugyo virus.
WHO’s own statement of the position is unambiguous: “We do not know whether this vaccine is efficacious against Bundibugyo disease in humans.”
Why “research only” is the harder choice
With doses available and people dying, restricting a vaccine to trial protocols looks perverse. The reasoning is worth following, because it is the opposite of caution for its own sake.
If Ervebo were deployed in a mass campaign and the outbreak subsequently receded, nobody could say whether the vaccine helped. Outbreaks end for many reasons — contact tracing, isolation, behaviour change, exhaustion of susceptible contacts. Without a comparison group, the data would be uninterpretable.
Worse, a vaccine that does not work against this species but is believed to work creates a specific danger: vaccinated health workers and contacts may reduce their precautions on the strength of protection they do not have.
Running it as a trial produces an answer. That answer then governs not only this outbreak but every future Bundibugyo outbreak — and there will be more.
WHO’s guidance (reference B09884, dated 31 August and published 1 September) supersedes an earlier version from 28 May 2026. It states that available evidence remains insufficient to determine whether Ervebo provides clinically meaningful protection against Bundibugyo virus disease, while noting that some findings suggest possible cross-protection.
A vaccination campaign began on 27 August 2026 using Ervebo within that research framework.
A change in trial design
There is a technical detail with real consequences. The planned trial uses individual randomisation of known contacts, rather than the group-based “ring vaccination” design used in previous Ebola vaccine trials.
Ring vaccination identifies a case, then vaccinates that person’s contacts and their contacts — a ring around the case. It was used to demonstrate Ervebo’s efficacy against Zaire ebolavirus and is operationally elegant, since it doubles as an outbreak-control measure.
Individual randomisation is statistically cleaner but ethically heavier: it means some identified contacts of confirmed cases are randomised not to receive the vaccine. Choosing it reflects how much genuine uncertainty exists here. With Zaire, ring vaccination was defensible partly because there was prior reason to expect benefit. Here, there is not.
The response on the ground
Contact tracing figures give a sense of the operation’s scale: 22,091 of 26,850 required contacts were seen within WHO’s 24-hour follow-up window.
That is roughly 82 per cent — a substantial achievement across sixty health zones in a country with limited transport infrastructure and areas of insecurity. It also means about 4,700 contacts were not seen on time, and in a disease with this transmission profile, missed contacts are how outbreaks continue.
Around 500,000 Ervebo doses are available through the International Coordinating Group on Vaccine Provision. Supply is not the binding constraint. Evidence is.
What is unresolved
Several things in the public record are less firm than they appear, and are flagged here rather than smoothed over:
- Trial enrolment figures circulating in secondary coverage could not be confirmed against a primary trial registry entry.
- Regulatory and ethics approval status in the DRC was reported as in progress in mid-August; its current state is not clearly documented publicly.
- Case counts in an active outbreak lag reality. Confirmed cases require laboratory confirmation, and deaths outside the health system are undercounted.
Why an outbreak of this size is not front-page news
Nearly 2,800 deaths, a PHEIC declaration, and exported cases in two countries would normally command sustained international attention.
Several factors work against it: the DRC has limited international press presence; affected regions are difficult and sometimes dangerous to reach; the technical distinction between viral species makes the story harder to summarise; and there is a documented pattern of attention to Ebola tracking perceived risk to wealthy countries rather than deaths.
One case has reached France. Twenty have reached Uganda. Both are early tests of containment.
What to watch
- Trial results. WHO says it needs robust efficacy data before its guidance can change. Until then, every dose given is also a data point.
- Phase 1 results for Oxford and Moderna candidates, expected in September 2026, which could widen the options available to the trial.
- The case fatality rate. Movement below 48 per cent would suggest improving supportive care or earlier presentation.
- Exported cases. Uganda and France are where containment is measured.
The six species, and why the difference is not academic
The genus Orthoebolavirus contains six recognised species, and four are known to cause disease in humans:
- Zaire ebolavirus — historically the deadliest and the cause of most large outbreaks, including West Africa in 2014–16. This is what Ervebo targets.
- Sudan ebolavirus — recurrent outbreaks in Uganda and South Sudan. No licensed vaccine.
- Bundibugyo virus — first identified in Uganda in 2007. The cause of the current outbreak. No licensed vaccine.
- Taï Forest virus — a single documented human case.
The species differ genetically by enough that immunity to one does not reliably confer immunity to another. Ervebo works by expressing the surface glycoprotein of Zaire ebolavirus, training the immune system to recognise that specific structure. Bundibugyo’s glycoprotein is related but distinct.
Cross-protection is a real biological possibility — and it has been observed in some animal work — but “possible in animals” and “protective in humans” are separated by exactly the kind of trial WHO is now insisting on. Vaccines that looked promising in primates have failed in people often enough that the field does not treat the inference as safe.
Why ring vaccination was the right design last time and not this time
Ring vaccination has an honourable history. It was central to smallpox eradication, and it was used to demonstrate Ervebo’s efficacy against Zaire ebolavirus in Guinea.
The method identifies a confirmed case and vaccinates their contacts, and the contacts of those contacts — a ring around the infection. It is efficient in an outbreak because it concentrates doses where transmission risk is highest, and it doubles as outbreak control.
The trial version randomises rings to immediate or delayed vaccination, comparing infection rates between the two groups.
Its weakness for this situation is statistical. Randomising by group rather than by individual reduces effective sample size and requires larger numbers to detect an effect. In the Guinea trial that was acceptable because prior evidence gave good reason to expect protection.
Here there is no such prior. If Ervebo does not protect against Bundibugyo virus, a design that struggles to detect a modest effect could produce an ambiguous result — the worst outcome, because it would leave the question open through the next outbreak too. Individual randomisation is statistically sharper. It is also harder to justify to a contact who is randomised to wait, which is precisely why the choice signals how uncertain the science is.
What treatment exists regardless of vaccination
Vaccines are not the only intervention, and the distinction matters for what a 48 per cent fatality rate means.
Monoclonal antibody treatments — developed against Zaire ebolavirus and shown to reduce mortality substantially in that context — face the same species-specificity problem as the vaccine. Antibodies bind to particular structures, and those structures differ.
Supportive care, however, is not species-specific and is not a minor factor. Ebola kills substantially through fluid and electrolyte loss and consequent organ failure. Aggressive fluid replacement, electrolyte correction, and management of complications reduce mortality regardless of viral species.
This is why case fatality rates for the same virus vary so widely between settings. The difference between a treatment centre with reliable supplies, laboratory monitoring and adequate staffing and one without is measured in survival. A fatality rate near 48 per cent reflects the virus, and it also reflects how early patients present and what is available when they do.
Contact tracing, and why the missed 18 per cent matters
Ebola transmission requires direct contact with body fluids. That is what makes it controllable in principle: it does not spread through the air, and an infected person is not contagious before symptoms appear.
The control method follows directly. Identify every contact of a confirmed case. Monitor them daily for 21 days — the maximum incubation period. Isolate anyone who develops symptoms before they can infect others.
Done completely, this stops an outbreak without any vaccine at all. That is how Ebola outbreaks were ended for decades.
The catch is that it must be close to complete. WHO’s figures — 22,091 contacts seen within the 24-hour window out of 26,850 required — represent roughly 82 per cent. The remaining 4,700 are where transmission continues.
The reasons contacts are missed are consistent across outbreaks: distance and poor roads, insecurity in parts of eastern DRC, and community mistrust rooted in real history — response teams arriving in protective equipment, taking sick relatives away, and returning bodies for burial in ways that conflict with local practice. Where trust fails, contacts are concealed, and the chain continues unobserved.
This is why WHO’s guidance repeatedly emphasises community engagement. It is not a public-relations addendum. It is the mechanism by which the other 18 per cent is reached.
This article reports public health guidance and is not medical advice. For travel or vaccination questions, consult your national health authority.
Sources
- WHO, “Emergency guidance on the use of licensed Ebola vaccine during Bundibugyo virus disease outbreaks,” published 1 September 2026 — who.int
- WHO Disease Outbreak News, “Ebola disease caused by Bundibugyo virus – Democratic Republic of the Congo,” updated 26 August 2026 — who.int
- STAT News, “Talks on launch of clinical trial of Ebola vaccine in DRC advance, WHO says,” 12 August 2026 — statnews.com
- WHO, “Ebola outbreak – DRC 2026” situation page — who.int
