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.

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.”