Context
Ervebo® is currently the only licensed Ebola vaccine available. Ervebo® (rVSV-EBOV-GP) is a live, recombinant vesicular stomatitis virus (rVSV)-based vaccine licensed that was prequalified by the World Health Organization (WHO) in 2019 for the prevention of Ebola virus disease caused by Ebola virus (EBOV, species Orthoebolavirus zairense, previously known as Zaire ebolavirus) in individuals aged one year or older. It is not licensed for use against Bundibugyo virus (BDBV) and therefore the use of Ervebo® against BDBV constitutes off-label use{1} BDBV and EBOV cause Ebola disease that is clinically similar but are genetically and antigenically distinct virus species. Their glycoproteins share approximately only 60–65% amino acid sequence identity, a distinction that is particularly relevant for vaccination because currently available Ebola vaccines, including Ervebo®, target the viral glycoprotein.
Consequently, although Ervebo® is highly effective against EBOV, the extent to which Ervebo®-induced immunity provides clinically meaningful protection against BDBV remains unknown. (1)
On 19 August 2026 (2), the Strategic Advisory Group of Experts on Immunization (SAGE) reviewed and deliberated on the additional evidence that had become available following the publication of the WHO emergency guidance on the use of licensed Ebola virus vaccine during Bundibugyo virus disease outbreaks, 28 May 2026. (3)
The BDBV outbreak in the Democratic Republic of the Congo has spread substantially, with continued transmission and high mortality, creating an urgent need to strengthen outbreak response and reduce preventable deaths. (4) The severity of the disease and evolving epidemiological situation increases the imperative to consider all potentially beneficial interventions, while at the same time carefully weighing the unknown efficacy of Ervebo® against BDBV, and resultant risks.
A ring vaccination randomized controlled trial (henceforth referred to as ring RCT) of Ervebo® and BDBV-specific vaccine candidates is planned to be conducted in the Democratic Republic of the Congo as a matter of urgency. (5) If well designed and rigorously implemented, the trial would provide the critical evidence currently lacking on the efficacy of Ervebo® against BDBV.
Assessment of additional evidence on Ervebo® performance against BDBV since May 2026
Findings from an increased, albeit still limited, number of studies conducted to date in nonhuman primates and ferrets suggest some protection by Ervebo® against BDBV-related mortality, while showing little or no protection against viraemia and clinical disease (...).
In the absence of an established correlate or surrogate of protection against BDBV, the extent to which findings from animal challenge models (disease and laboratory-based immunological studies) can predict protection in humans remains unknown.
Available human immunogenicity data regarding potential cross-protection conferred by Ervebo® against BDBV showed that Ervebo® induces cross-reactive BDBV glycoproteinbinding antibodies which are at levels approximately 5-fold lower than against EBOV, and that cross-neutralization of BDBV pseudoviruses with neutralizing titres are approximately 3.5–4-fold lower than against EBOV (...).
No Ervebo® vaccine efficacy data against BDBV in humans have been generated yet.
There are a small number of anecdotal reports of previously vaccinated health care workers who subsequently developed BDBV disease and survived. However, no conclusions regarding vaccine efficacy or effectiveness can be drawn from these observations because of the very small sample size, the absence of an appropriate comparator group, the potential for substantial bias, non-systematic data collection, and the resulting considerable statistical uncertainty.
Overall, the limited data and anecdotal reports are suggestive of some protection against BDBV-related mortality and are consistent in trending towards some as yet unquantified benefit.
In conclusion, the available data remain insufficient to determine whether Ervebo® provides any clinically meaningful protection against BDBV in humans or to reliably estimate the magnitude of such protection, including protection against infection, disease, severe disease or death.
Benefit–risk considerations regarding off-label use of Ervebo® in the context of unknown efficacy against BDBV
There is currently clinical equipoise regarding the efficacy of Ervebo® against BDBV.
Vaccine efficacy in humans remains to be established and could range from high efficacy, with substantial clinical and public health benefit, through moderate or partial efficacy, to limited or negligible efficacy, with little or no meaningful protection. The lower limit of the range of potential effects may even include harm. The potential ratio of benefits and risks of broader use therefore differ considerably depending on where within this range the true efficacy ultimately lies.
If efficacy is high or clinically meaningful, broader use while the ring RCT is underway could potentially reduce severe disease and deaths and, if the vaccine also protects against infection and transmission, contribute to outbreak control. In a context where other medical countermeasures remain limited, earlier access could provide populations at high risk with a vaccine that has a well-characterized safety profile, while BDBV-specific vaccines remain under evaluation and are not yet available for use. Use within appropriately designed research frameworks could also generate complementary real-world effectiveness data. In addition, vaccinated individuals would be expected to benefit from protection against Ebola virus disease should they subsequently be exposed during a future EBOV outbreak.
Conversely, if efficacy is low, negligible or absent, the balance of benefits and risks would be substantially less favourable. Considerable financial, logistic and human resources would be diverted to an intervention providing little or no clinical or public health benefit. These resources could otherwise support outbreak-control measures of established effectiveness, including surveillance, contact tracing, timely testing and case detection, isolation, infection prevention and control, and safe and dignified burials. These resources could also have been better invested in the development of BDBV-specific vaccines.
A scenario in which Ervebo® provides meaningful protection against severe disease or death, but limited or no protection against infection, viraemia or onward transmission, would require careful consideration. Protection against severe disease or death would constitute an important individual and public health benefit, even in the absence of substantial effects on infection or transmission. However, vaccination could then reduce morbidity and mortality without necessarily interrupting transmission. If this efficacy profile is not clearly understood and communicated, vaccination could lead to false reassurance among vaccinated individuals, communities and responders, potentially reducing adherence to established outbreak-control measures. Such behavioural changes could offset some of the benefits of vaccination and, if infection and onward transmission are not sufficiently reduced, could contribute to continued transmission and potentially prolong or exacerbate the outbreak.
If efficacy proves limited or negligible, substantial numbers of breakthrough cases or deaths could also undermine public trust in the outbreak response and confidence in Ebola vaccines, vaccination programmes generally, and the health sector more broadly. Once broader vaccination has commenced, a subsequent decision to restrict or discontinue Ervebo® use, if the evidence shows limited efficacy, could itself create important communication and trust challenges.
There are also important evidence-generation trade-offs. The use of Ervebo® outside rigorous research protocols could interfere with the feasibility, recruitment, implementation and scientific integrity of studies, particularly the ring RCT, designed to establish vaccine efficacy for Ervebo® and BDBV-specific vaccine candidates (which are expected to have the potential for better performance against BDBV). This could delay the generation of the robust evidence needed to guide policy. Observational Ervebo® effectiveness studies could provide useful complementary information but are inherently more susceptible to bias and confounding than RCTs and may therefore be more difficult to interpret or insufficiently robust to resolve the central question of efficacy. Vaccine effectiveness (VE) studies of vaccines with modest efficacy are particularly prone to these limitations. Hence the value of any observational study depends partly on the extent to which its design can ensure that uptake of the intervention is as close to random as possible and that outcome data are collected systematically from all participants allowing comparable analysis, thereby reducing selection and information biases. Conversely, if the ring RCT demonstrates clinically meaningful efficacy of Ervebo® and/or BDBV-specific vaccine candidates, such a trial would provide a strong basis for rapidly updating policy and expanding vaccine(s) use to benefit the wider population.
Extensive reliance on Ervebo® could potentially affect community willingness to participate in future studies or receive BDBV-specific vaccines, which become especially important if the efficacy of Ervebo® against BDBV is insufficient.
Finally, widespread deployment would have implications for global vaccine security. Largescale use of available Ervebo® doses against BDBV could temporarily deplete the International Coordinating Group on Vaccine Provision stockpile and potentially compromise timely access to vaccine for response to a future outbreak caused by EBOV, against which Ervebo® has demonstrated efficacy and is licensed.
Taken together, the uncertainties described above reinforce the importance of obtaining robust efficacy data as rapidly as possible while carefully weighing the potential benefit of any broader use against its potential negative consequences.
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{1} Use of a vaccine for an unapproved indication (not described in the approved labelling) or in an unapproved age group, dosage, or route of administration.
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© World Health Organization 2026. Some rights reserved. This work is available under the CC BY-NC-SA 3.0 IGO licence.
Suggested citation. WHO emergency guidance on the use of licensed Ebola vaccine during Bundibugyo virus disease outbreaks, 31 August 2026. Geneva: World Health Organization; 2026. https://doi.org/10.2471/B09884
Source:
Link: https://doi.org/10.2471/B09884
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