The World Health Organization (WHO) has recommended prioritising the Ervebo vaccine for testing in a clinical trial in the Democratic Republic of the Congo. The vaccine was originally developed to protect against Zaire ebolavirus, the species responsible for the devastating west African epidemic just over a decade ago. It was not designed for the Bundibugyo species responsible for the current outbreak – which raises the question of why scientists are considering it at all.
The current Ebola outbreak was declared a public health emergency of international concern in May 2026. By August 9, there had been 4,402 confirmed cases and 2,013 deaths. All but 21 of these cases were reported in the Democratic Republic of the Congo (DRC), with 20 cases in Uganda and one in France.
These figures make the current outbreak the second-largest Ebola epidemic on record, surpassed only by the west African epidemic of 2014–16, where 11,000 people died.
At present, there are around 75–100 new cases being reported each day. The UN has described it as the fastest-growing Ebola outbreak ever recorded. Tedros Adhanom Ghebreyesus, the director general of the WHO, said the current outbreak is on track to become the world’s largest.
Unfortunately, there is still no clear evidence that the outbreak is coming under control, and the number of infections may still be rising rapidly.
Ebola outbreaks are usually brought under control through a combination of vaccination and public health measures. During an outbreak, vaccines can be particularly effective, especially when used quickly.
Health authorities may carry out large-scale vaccination campaigns. They can also use a strategy known as “ring vaccination”. This involves offering the vaccine to people who have been in close contact with someone who has been infected. The aim is to create a protective ring around each case and stop the virus from spreading further.
Public health measures are also crucial. These include finding cases early, tracing contacts and moving infected people to specialist treatment centres where they can be cared for safely and isolated from others until they recover. Caring for someone who is seriously ill with Ebola carries a particularly high risk of infection.
Public education is also important, including advice on caring for patients safely and reducing the risk of transmission during funerals.
No licensed vaccine
The challenge is that, unlike the Zaire species of Ebola, there is currently no licensed vaccine for Bundibugyo virus disease. There are also no licensed vaccines for some other Ebola species, including Sudan virus disease and Taï Forest virus disease. As a result, vaccination has not yet played a role in controlling the outbreak.
Several Bundibugyo vaccines are under development. Two have already entered phase 1 clinical trials. One has been developed by Oxford University and the Serum Institute of India, while another is being developed by Moderna.
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A phase 1 clinical trial is the first stage of testing a new vaccine in people. These studies are mainly designed to assess safety and identify any side-effects. It is not yet clear how long it will take before these vaccines could become widely available if they prove successful. Neither are being tested in Africa.
Without a vaccine, efforts to control the outbreak depend entirely on public health measures. But putting these measures into practice can be difficult, especially during a fast-moving outbreak.
Fear and uncertainty are common during outbreaks of dangerous diseases. Misinformation and rumours can spread quickly. During the west African Ebola epidemic, reports suggested that some families hid sick relatives because they feared they would be taken away by health authorities. In some settings, there have also been reports of mistrust towards official responses to outbreaks and concerns about the motives of outside organisations involved in disease control efforts.
The current situation in the DRC is made even more difficult by the ongoing armed conflict in some of the affected areas. Conflict can make it much harder for health workers to reach communities, track cases and implement control measures.
There have already been several attacks on Ebola treatment centres in the country this year. In one incident, a treatment centre was set on fire after a dispute over the handling of the body of an infected person.
Another worrying sign is that many newly diagnosed patients had not previously been identified as contacts of earlier cases. This suggests that the virus may be spreading unnoticed in some communities.
Against this backdrop, the WHO advisory group has recommended testing the Ervebo vaccine.
Ervebo was developed to protect against the Zaire species of Ebola and is estimated to be around 84% effective against that species from ten days after a single dose.
Why scientists are considering using it in the current outbreak
The idea that it might also help against Bundibugyo has been raised before. In 2011, researchers found that a single dose protected monkeys from dying after they were later exposed to the Bundibugyo virus. Animals that survived also developed less severe illness and had lower levels of virus in their bodies.
Researchers have also found signs that the vaccine stimulates an immune response against Bundibugyo in people. In 2019, scientists reported that people who received Ervebo produced antibodies that recognised the Bundibugyo virus, although the response was weaker than the one seen against the Zaire species. A more recent study has confirmed these findings.
That leaves an open question: how well Ervebo actually works against Bundibugyo in the real world.
The available evidence suggests that Ervebo can trigger an immune response against the Bundibugyo virus. However, scientists still do not know how well it works against this species in real-world conditions.
The animal studies provide reasons for optimism. If the vaccine offers even partial protection, it could reduce the number of deaths. By lowering the amount of virus in infected people, it might also help reduce transmission.
Researchers also need to investigate whether additional doses could strengthen protection.
For now, the important question is whether Ervebo can provide meaningful protection while purpose-built Bundibugyo vaccines are still being developed. With the outbreak continuing to grow and control efforts facing significant difficulties, clinical trials could provide urgently needed answers.
Paul Hunter consults for the World Health Organization. He receives funding from National Institute for Health Research and has received funding from the World Health Organization and the European Regional Development Fund