July 24, 2026

Is the licensed Zaire ebolavirus vaccine suitable to combat the current Bundibugyo ebolavirus outbreak?

Researchers at the German Primate Center – Leibniz Institute for Primate Research in Göttingen provide evidence of possible cross-protection through an already licensed Zaire ebolavirus vaccine.

Medical staff in white protective suits. The central figure is adjusting her face shield over her mask and gloves. On the right, another person is also adjusting her visor. — The current outbreak of the Bundibugyo Ebola virus in the Democratic Republic of the Congo and Uganda is the largest outbreak of this virus variant ever documented.

Ebolaviruses are among the most dangerous known human pathogens. Three ebolavirus species have repeatedly caused major disease outbreaks on the African continent. Most outbreaks have been caused by Zaire ebolavirus. Vaccines and therapeutics are available for this virus species. In contrast, no therapeutics or vaccines are currently available against Bundibugyo ebolavirus, which is responsible for the ongoing outbreak in the Democratic Republic of the Congo and Uganda. This significantly complicates efforts to control the outbreak, which is already the largest documented Bundibugyo ebolavirus outbreak to date. It was previously unclear whether the currently circulating virus variant can enter human cells more efficiently than earlier variants and thereby contribute to the unusual dynamics of the outbreak. It was also unknown whether licensed vaccines against Zaire ebolavirus can provide at least partial protection against Bundibugyo ebolavirus. A research team from the German Primate Center – Leibniz Institute for Primate Research (DPZ), together with scientists from the TWINCORE – Centre for Experimental and Clinical Infection Research, the University Medical Center Hamburg-Eppendorf, and the Charité – University Hospital Berlin, investigated both questions. The researchers found no evidence that the current Bundibugyo ebolavirus variant infects human cells more efficiently. Viral entry into cells could be inhibited by antibodies present in sera from individuals vaccinated with the vector vaccine VSV-ZEBOV-GP. The study therefore provides important evidence that an already available vaccine might protect individuals at risk of severe disease and contribute to combat the ongoing outbreak (The Lancet Infectious Diseases).

Largest documented Bundibugyo ebolavirus outbreak to date

Since the first infections with Bundibugyo ebolavirus were reported in the northeast of the Democratic Republic of the Congo in May 2026, the outbreak has spread further and reached neighboring Uganda. According to the World Health Organization (WHO) as of 17 July 2026, 2,124 infections have been confirmed to date and 828 people have died as a result of the infection. An end to the spread is currently not foreseeable. “This is already the largest documented outbreak of this ebolavirus species to date,” explains Stefan Pöhlmann, head of the Department of Infection Biology at the DPZ.

No licensed therapeutics or vaccines available against Bundibugyo ebolavirus

Bundibugyo ebolavirus is one of four ebolavirus species that can cause disease in humans. Repeated major outbreaks have so far been caused primarily by Zaire ebolavirus, including the 2014-2016 West-African Ebola epidemic, which resulted in more than 28,000 infections and over 11,000 deaths. Licensed vaccines and therapeutics against Zaire ebolavirus are now available. Such medical countermeasures are lacking for infections with Bundibugyo or Sudan ebolavirus. Their development and clinical evaluation will still require time, which significantly complicates efforts to control the current outbreak.

No evidence for increased infectivity of the Bundibugyo ebolavirus variant

Because the Zaire ebolavirus variant responsible for the largest Ebola epidemic in West-Africa in 2014- 2016 carried a mutation in the glycoprotein that likely facilitated entry into human cells, the researchers first investigated whether the current Bundibugyo ebolavirus variant enters human cells more efficiently than previous variants. For their studies, the researchers at the German Primate Center produced so-called pseudoviruses, replication-incompetent virus particles equipped with the glycoproteins of different ebolaviruses. These pseudoviruses were subsequently used to infect macrophages and dendritic cells, which are the main target cells of ebolaviruses. The analyses provided no evidence of increased infectivity of the currently circulating Bundibugyo ebolavirus variant.

Two scientists in white lab coats stand in a laboratory, posing beside scientific equipment, including a microscope and a large instrument. A colorful fluorophore selection guide is visible on the wall behind them. — Infection biologist Dr. Markus Hoffmann (left) and Prof. Dr. Stefan Pöhlmann, head of the Infection Biology Unit at the German Primate Center (DPZ) – Leibniz Institute for Primate Research.

Antibodies generated by vaccination against Zaire ebolavirus also neutralize the current Bundibugyo ebolavirus variant

To investigate whether licensed vaccines against Zaire ebolavirus also induce antibodies capable of neutralizing the currently circulating Bundibugyo ebolavirus variant, the researchers mixed pseudoviruses with serum samples from vaccinated individuals who had participated in a previous clinical study on the immunogenicity and tolerability of the VSV-ZEBOV-GP vaccine. They subsequently analyzed the extent to which the antibodies present in the sera inhibited pseudovirus entry into human cells. The results showed that most samples contained antibodies that neutralized pseudoviruses carrying the glycoprotein of the current Bundibugyo ebolavirus variant, although less efficiently than pseudoviruses carrying the Zaire ebolavirus glycoprotein.

Further studies are required to confirm clinical benefit during the current outbreak

“Our results show that the currently circulating Bundibugyo ebolavirus variant does not have an increased potential to infect human cells compared with previous variants. At the same time, our data suggest that the available vaccine against Zaire ebolavirus may also contribute to combat the current Bundibugyo ebolavirus outbreak until specific vaccines and therapeutics become available,” says Markus Hoffmann, head of the study and first author. However, he adds: “Our results need to be confirmed in studies using infectious Bundibugyo ebolavirus. In addition, further investigations are required to reliably assess the clinical benefit of using Zaire ebolavirus vaccines during the current outbreak.”

Update from 11.08.2026:

Based on a study by researchers at the DPZ, as well as other recent studies that have also yielded encouraging findings, the WHO has reconsidered its position and now recommends the use of the Zaire ebolavirus vaccine as part of a clinical trial in the Democratic Republic of the Congo. At the same time, specific vaccine candidates against the Bundibugyo ebolavirus are currently being tested intensively in safety studies with healthy volunteers and could be available in just a few months.
Further information: “Mismatched” vaccine being rolled out against Ebola outbreak. DOI: 10.1126/science.z7h4e51

Since Uganda has not had any new cases for some time, the outbreak there has been declared over.

Info boxes

Info box 1

The Viral Glycoprotein Is the Key to Cell Entry of Ebolaviruses

Ebolaviruses typically enter the body through contact with bodily fluids of infected humans or animals. Microscopic injuries to the skin, known as microlesions, serve as entry points. Among the first target cells of the virus are macrophages and dendritic cells – specialized immune cells in which the virus initially replicates. From there, it spreads throughout the body via the bloodstream, infecting additional organs and cell types. A central role in this process is played by the viral glycoprotein. It binds to the surface of the host cell and is subsequently taken up into membrane-bound vesicles. Within these vesicles, the glycoprotein becomes active again and mediates the fusion of the vesicle membrane with the viral envelope. This allows the viral genetic material to enter the interior of the cell, where it serves as the blueprint for the production of new virus particles.

Info box 2

An Earlier Study with Non-Human Primate Suggests Possible Cross-Protection of Zaire Ebolavirus Vaccines Against Bundibugyo Ebolavirus

Due to the high pathogenicity of ebolaviruses, vaccines and therapeutics have been under development for many years. However, much of this work has been carried out in academic research settings, as Ebola outbreaks have generally been geographically limited and involved relatively small numbers of cases. As a result, the development of such medical countermeasures has offered only limited commercial incentives for the pharmaceutical industry. Nevertheless, significant progress has been made in recent years. The first licensed vaccine against Zaire ebolavirus is the result of many years of basic research, including preclinical studies in non-human primate models. Following successful clinical safety trials in humans, the vaccine was deployed to help combat the 2014-2016 West-African Ebola epidemic. In addition, a previous preclinical study by scientists from the US demonstrated that a Zaire ebolavirus vaccine protected a proportion of non-human primates from fatal disease following challenge with an earlier variant of Bundibugyo ebolavirus (https://doi.org/10.1093/infdis/jir350). These findings provided the first evidence of possible cross-protection.