Zaire Ebola vaccine may partly protect against Bundibugyo outbreak

A Zaire Ebola vaccine may offer partial protection against Bundibugyo ebolavirus during the current outbreak in the Democratic Republic of the Congo and Uganda, according to researchers reporting in The Lancet Infectious Diseases. The finding matters because no licensed vaccine or therapeutic is currently available specifically for Bundibugyo ebolavirus, according to the Leibniz Institute for Primate Research.

The study, led by scientists at the German Primate Center—Leibniz Institute for Primate Research with partners at TWINCORE, University Medical Center Hamburg-Eppendorf and Charité—University Hospital Berlin, tested whether antibodies produced after vaccination with VSV-ZEBOV-GP could block cell entry by particles carrying Bundibugyo virus surface proteins.

The researchers found that most serum samples from vaccinated people contained antibodies that inhibited entry of pseudoviruses carrying the glycoprotein from the current Bundibugyo variant. The effect was weaker than against pseudoviruses carrying the Zaire ebolavirus glycoprotein, the team reported.

Does the Zaire Ebola vaccine work against Bundibugyo?

The study suggests the licensed Zaire ebolavirus vaccine could provide some cross-protection against Bundibugyo ebolavirus, but it does not prove that vaccinated people are protected during an outbreak. The researchers said confirmation will require studies using infectious Bundibugyo ebolavirus and further work to measure any clinical benefit.

VSV-ZEBOV-GP is designed against Zaire ebolavirus, the species behind most major Ebola outbreaks and the 2014-2016 West African epidemic. Bundibugyo ebolavirus is a different species that can also cause human disease, and the current outbreak has exposed the lack of species-specific medical countermeasures, according to the research team.

What researchers found about the current variant

The team also examined whether the Bundibugyo variant now circulating might enter human cells more efficiently than earlier variants. That question was raised because a Zaire ebolavirus variant involved in the 2014-2016 epidemic carried a glycoprotein mutation that likely helped it enter human cells, according to the researchers.

For the new work, the German Primate Center team used pseudoviruses, which are replication-incompetent particles fitted with glycoproteins from different ebolaviruses. They tested entry into macrophages and dendritic cells, described by the researchers as main target cells for ebolaviruses.

The analyses found no evidence that the current Bundibugyo variant has a greater ability to infect human cells than previous variants. Markus Hoffmann, the study leader and first author, said the data indicate the current variant does not show increased cell-infection potential compared with earlier variants, while also suggesting the available Zaire vaccine may help until Bundibugyo-specific vaccines and treatments exist.

How large is the Bundibugyo Ebola outbreak?

The outbreak began with infections reported in northeastern Democratic Republic of the Congo in May 2026 and later reached Uganda, according to the Leibniz Institute for Primate Research. The World Health Organization reported 2,124 confirmed infections and 828 deaths as of July 17, 2026.

Stefan Pöhlmann, head of the Department of Infection Biology at the German Primate Center, said the outbreak is already the largest documented outbreak of this ebolavirus species. The institute said the spread had not yet shown a foreseeable end as of the report.

The authors cautioned that the laboratory findings are not enough to settle vaccine policy for the outbreak. Hoffmann said studies with infectious Bundibugyo ebolavirus and additional clinical investigations are needed before researchers can reliably judge whether Zaire ebolavirus vaccines benefit people at risk during the current outbreak.

This story draws on original reporting from Medical Xpress.