Bundibugyo Ebola outbreak: first findings to guide the development of an effective vaccine
Paris, 24 July 2026
Publié le 24 July 2026 (dernière mise à jour le 24 July 2026)
In brief
Less than three months after the WHO declared an Ebola outbreak caused by the Bundibugyo virus in Central Africa, researchers from Inserm and the University of Bordeaux, based at the Vaccine Research Institute (VRI) and the Bordeaux Population Health Centre, in collaboration with the French National Research Institute for Sustainable Development (IRD), have reported initial findings that could accelerate the development of an effective vaccine. Analysis of serum samples from participants vaccinated with the vaccines currently authorised against the Zaire Ebola strain showed that they had developed antibodies against the Bundibugyo strain. Although these findings do not demonstrate clinical protection against the Bundibugyo strain, they provide the first human immunological data that could help inform decisions on the outbreak response. In particular, the findings support the evaluation of the Ebola vaccines currently available while strain-specific vaccines remain under development. The emergence of this new strain also highlights the importance of developing vaccines that provide broad protection against multiple Ebola strains.
Beyond these findings, the scientific evidence published today has been made possible by nearly 15 years of investment by Inserm and its partners in the fight against Ebola virus disease.
These findings are published in The New England Journal of Medicine.
Improving the response to Ebola outbreaks
The current Ebola virus disease outbreak caused by the Bundibugyo strain in the Democratic Republic of the Congo and Uganda has spread rapidly, with more than 1,000 deaths reported as of 22 July 2026.
As the outbreak continues, there are currently no approved vaccines or specific treatments against this newly emerging strain. In response to this public health emergency, experts at the World Health Organization (WHO) have recommended evaluating the effectiveness of different therapeutic and preventive strategies to improve the response to Ebola outbreaks. In this context, ANRS Emerging infectious diseases and Inserm are implementing a comprehensive research strategy.
Existing vaccines tested against the Bundibugyo Ebola virus strain
As one of the first projects within this comprehensive research strategy, an Inserm-led study investigated whether vaccines already authorised against the Zaire strain of the Ebola virus could also provide protection against the Bundibugyo strain. To do so, the researchers analysed serum samples from 179 participants enrolled in the PREVAC clinical trial (see below), conducted in West Africa between 2018 and 2020. They assessed the immune responses of adults and children who had previously received either the rVSV-ZEBOV vaccine (Ervebo®, MSD) or the Ad26/MVA-BN-Filo vaccine regimen (Zabdeno®, Mvabea®, Janssen), by measuring the levels of antibodies produced against different Ebola virus strains, including the Bundibugyo strain.
Results: Both vaccination strategies induced early, detectable immune responses against the Bundibugyo strain, regardless of participants’ age or the timing of sample collection (28 days or three months after vaccination). However, the immune response against the Bundibugyo strain was weaker than that observed against the Zaire strain, the original target of these vaccines. Although it was not possible to assess vaccine effectiveness directly against the Bundibugyo strain, these findings support the biological plausibility of partial immunity following vaccination against the Zaire Ebola strain.
In the absence of Bundibugyo-specific licensed vaccines, these findings support the urgent clinical evaluation of the vaccines currently available—particularly Ervebo, given the existing vaccine stockpile and its single-dose regimen—in the context of the ongoing outbreak. They also, and above all, highlight the importance of developing pan-filovirus vaccines capable of providing broad protection against multiple Ebola virus strains.
A long-term investment for the benefit of vaccine research
Beyond these initial findings, this work demonstrates the importance of sustained investment in clinical vaccine research against Ebola. Without the preservation of serum samples collected from trial participants, this study could not have been carried out so rapidly following the emergence of the Bundibugyo strain.
The scientific evidence presented today has been made possible by investments made over many years in the international PREVAC/PREVAC-UP consortium.
About PREVAC trial (Partnership for Research on Ebola Vaccinations)
PREVAC (Partnership for Research on Ebola Vaccinations) was a Phase II clinical trial conducted between 2018 and 2020 in Guinea, Liberia, Mali and Sierra Leone to evaluate the safety and immunogenicity of three Ebola vaccination strategies in healthy adults and children. Participants were randomly assigned to receive one of the following vaccination regimens:
- The first regimen consisted of one dose of the Ad26.ZEBOV vaccine followed 56 days later by one dose of MVA-BN-Filo.
- The second regimen consisted of a single dose of the rVSVΔG-ZEBOV-GP vaccine.
- The third regimen consisted of one dose of the rVSVΔG-ZEBOV-GP vaccine followed by a booster dose of the same vaccine 56 days later.
The project was co-funded by Inserm, the National Institute of Allergy and Infectious Diseases (NIAID), the London School of Hygiene & Tropical Medicine (LSHTM) and the College of Medicine and Allied Health Sciences (COMAHS), with additional support from Guinea, Liberia, Mali and Sierra Leone. The field support provided by the NGO ALIMA was also instrumental in promoting community engagement in the research and ensuring long-term follow-up of the volunteers. Merck and Janssen supplied the vaccines used in the trial.
The project also received additional funding to support long-term follow-up of participants for up to five years after vaccination through PREVAC-UP, a project coordinated by Inserm. This extension was funded by the European & Developing Countries Clinical Trials Partnership (EDCTP2) programme, with support from the European Union.
As part of the overall research strategy led by ANRS Emerging infectious diseases and Inserm, the EBO-PEP clinical trial is also underway. The trial is evaluating the effectiveness of a post-exposure prophylaxis strategy in individuals at high risk of developing Ebola virus disease following exposure. It is being conducted in partnership with the National Institute for Biomedical Research (INRB) and the humanitarian organisation ALIMA (The Alliance for International Medical Action).
Sources
Cross-reactive Bundibugyo antibody responses after licensed Ebola vaccines
Edouard Lhomme M.D., Ph.D.,1 Aurélie Wiedemann Ph.D.,2 Ahidjo Ayouba Ph.D.,3 Safaa Ben-Farhat M.Eng.,4 Guillaume Thaurignac MSc,3 Céline Roy Ph.D,4 Abdoul Habib Beavogui M.D.,5 Seydou Doumbia M.D., Ph.D.,6 Mark Kieh M.D., M.S.M.H.C.,7 Bailah Leigh M.D.,8 Samba Sow M.D,9 Stephen A Migueles, MD,10 Deborah Watson-Jones M.D., Ph.D, 11 Yazdan Yazdanpanah M.D., Ph.D,12 Rodolphe Thiébaut, M.D., PhD., 1 Martine Peeters Ph.D.,3 Laura Richert M.D.,1 Yves Levy M.D., Ph.D.2 , on behalf of the PREVAC study Team *
Drs. Lhomme and Wiedemann and Drs. Richert and Levy contributed equally to this article.
* A complete list of members of the PREVAC Study Team is provided in the Supplementary Appendix.
- Univ. Bordeaux, F-33000 Bordeaux, France
- INSERM, 75013 Paris, France
- Institut de Recherche pour le Développement, 34090, Montpellier, France
- INSERM, F-33000 Bordeaux, France
- Centre National de Formation et de Recherche en Santé Rurale de Maferinyah, Maferinyah, Guinea
- University Clinical Research Center (UCRC), Bamako, Mali
- Partnership for Research on Ebola Virus in Liberia (PREVAIL), Monrovia, Liberia
- University of Sierra Leone, Freetown, Sierra Leone
- University of Maryland School of Medicine, 685 West Baltimore Street Baltimore, MD 21201-1509, USA
- National Institute of Allergy and Infectious Diseases, Bethesda, MD, USA
- London School of Hygiene & Tropical Medicine, London, UK
- AP-HP, Paris F-75018, France
The New England journal of medicine
DOI : 10.1056/NEJMc2608018
