New data suggests existing vaccines may help blunt the Bundibugyo outbreak
Key takeaways
- A study published on Wednesday in the New England Journal of Medicine (NEJM) provides evidence that Merck’s licensed Ebola vaccine may offer protection against the Bundibugyo ebolavirus species.
- The current outbreak in the Democratic Republic of the Congo (DRC) has caused more than 1,000 deaths and 2,536 confirmed cases since it was declared on May 15.
- Ervebo, the vaccine in question, was originally approved to target the Zaire ebolavirus species, but researchers suggest it generates antibodies that can react to the Bundibugyo strain.
- The Africa Centers for Disease Control and Prevention (Africa CDC) is planning a trial using a two-dose regimen to test this cross-protection in the field.
- The World Health Organization (WHO) has reportedly shown lukewarm support for testing the Merck vaccine in this context, while strain-specific vaccines remain months away from clinical trials.
Metrics of the current Ebola crisis in the Congo
| Metric | Detail |
|---|---|
| Confirmed Cases | 2,536 |
| Total Deaths | More than 1,000 |
| Outbreak Start Date | May 15 |
| Vaccine Intervention | Ervebo (Merck) |
| Primary Target Strain | Zaire ebolavirus |
| Current Outbreak Strain | Bundibugyo ebolavirus (BDBV) |
| Historical Vaccination Scale | >300,000 people (2018-2020 outbreak) |
Clinical evidence for cross-species protection
New human data published on Wednesday in the NEJM indicates that Merck’s Ervebo vaccine might assist in controlling the rapidly expanding Ebola outbreak in the northeastern DRC. The study, led by Edouard Lhomme, a professor at the University of Bordeaux, suggests that the immune response triggered by a vaccine designed for one species can potentially shield against another. This concept, known as cross-protection, is gaining traction as the current outbreak moves faster than health officials can track it.
The research arrived at a critical moment. While Ervebo is a licensed product, it is specifically indicated for the Zaire ebolavirus. The current emergency involves the Bundibugyo ebolavirus, a distinct species for which no approved vaccines or treatments exist. Researchers analyzed blood samples from previously vaccinated individuals to determine if antibodies produced to fight the Zaire strain would recognize and bind to the Bundibugyo strain. The findings showed that these cross-reactive antibodies are present, providing a scientific basis for testing the vaccine in the current crisis.
Understanding the virus and the vaccine platform
Ebola virus disease is a severe and often fatal illness. Most major outbreaks in the past, including the second-largest on record in the DRC between 2018 and 2020, were caused by the Zaire species. During that period, more than 300,000 people received the Ervebo vaccine. The Bundibugyo species is less common but equally dangerous. It is currently spreading through the northeastern regions of the DRC, creating a gap in public health preparedness.
The mechanism of the vaccine is similar to a key designed for a specific lock. While the Ervebo “key” was cut precisely for the Zaire “lock,” its shape is close enough that it might still turn the Bundibugyo “lock” and prevent the virus from entering human cells. This hypothesis was first explored roughly 10 years ago in animal studies. Those trials showed increased survival in primates that were given vaccines for Zaire and Sudan species and then exposed to a lethal dose of the Bundibugyo virus.
The high stakes of the Congo outbreak
The scale of the current outbreak is significant. Since the DRC Ministry of Health declared the outbreak on May 15, the death toll has climbed past 1,000. Health officials reported 2,536 cases, a figure that highlights the speed of transmission. For comparison, the West Africa outbreak of 2013-2016, which resulted in more than 11,000 deaths, took approximately eight months to reach the 1,000-death milestone. The current spread is moving even more aggressively.
Dr. Jean Kaseya, head of the Africa CDC, spoke about the desperation on the ground during a health summit in Ghana. He noted that people are dying because of a lack of vaccines, medicines, and funding. The absence of an approved Bundibugyo-specific vaccine leaves the population vulnerable. If the Merck vaccine provides even partial protection, it could serve as an immediate tool to curb deaths while strain-specific candidates undergo development.
Read it with the right caution
The findings in the NEJM paper do not prove that Ervebo will definitely stop the Bundibugyo outbreak. The study provides evidence of an immune response in blood samples, but this does not always translate to clinical efficacy in a real-world setting. The evidence is currently considered additional support rather than a guarantee of protection.
The WHO has expressed “lukewarm support” for testing the vaccine in the DRC. This hesitation likely stems from the fact that any use of Ervebo against the Bundibugyo strain is considered off-label and experimental. Furthermore, the NEJM paper was posted online before peer-review, though it was subsequently published in the journal. It is also important to note that vaccines specifically designed for Bundibugyo are still described as being “months away” from clinical trials, leaving a significant window where no targeted medical countermeasure is available.
Ongoing efforts and trial timelines
The next step in this scientific inquiry is a prospective clinical study. Edouard Lhomme stated that the question of cross-protection should no longer be debated but rather tested in a carefully designed trial. The Africa CDC is moving forward with plans for a trial in the DRC that will likely utilize a two-dose regimen. This study aims to determine how effective the Zaire-targeted vaccine is when faced with the Bundibugyo species in an active outbreak zone.
In addition to the Merck vaccine, other experimental platforms are being monitored. The organization IAVI is developing an investigational vaccine for the Sudan ebolavirus using the same viral-vector platform as Ervebo. As health officials scale up surveillance and contact tracing, the international community faces a choice between waiting for strain-specific vaccines or deploying existing tools based on the new immune-response data. Funding remains a primary barrier to these efforts, as the current resources for vaccine procurement and field testing are reportedly insufficient to match the speed of the virus.
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Sources:
- STAT, “New study supports testing of Merck’s Ebola vaccine in DRC outbreak” (https://www.statnews.com/2026/07/22/new-study-supports-testing-of-mercks-ebola-vaccine-in-drc-outbreak/)
- The Week, “Congo Ebola deaths top 1000” (https://theweek.com/health/congo-ebola-deaths-top-1000)
- STAT, “Health Policy News Today: DC Diagnosis Newsletter” (https://www.statnews.com/2026/07/23/health-policy-news-today-dc-diagnosis-newsletter-john-wilkerson-2/)
Disclaimer: This article is for general information only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider about any medical condition or before making health decisions.