WHO warns against use of Kenya-tested Ebola virus vaccine in current outbreak

The WHO said the Ervebo vaccine can only be used in carefully controlled clinical trials.

The vaccine, which is used to protect against Ebola caused by the Zaire strain, was partly tested in Kenya from 2016, where about 40 healthy adult volunteers took part in early safety studies in Kilifi county.

Those trials were designed to check whether the vaccine is safe and whether it can stimulate an immune response in the body, not whether it prevents disease during outbreaks.

WHO said the vaccine should not be assumed to work against a different Ebola type known as the Bundibugyo virus, which is driving current outbreaks in the Democratic Republic of the Congo and Uganda.

Bundibugyo virus disease is caused by a different species of the Ebola virus from the Zaire strain that Ervebo targets. Scientists said this difference is important because vaccines are often designed to match a specific virus type, and protection against one strain does not always extend to another.

“Ervebo is not licensed for prevention of BVD and evidence on cross-protection to other Ebola virus species remains limited and inconclusive. WHO recommends that Ervebo should not be used outside carefully designed research settings, to allow for its performance against BDV to be assessed.”

The global health agency added that all Ebola vaccines and treatments identified for the current outbreak must only be used within clinical trials.

“The WHO advisory groups recommended that all the products identified and considered be used exclusively within clinical trials to generate robust data and ensure safe, ethical, and effective research,” WHO said.

The guidance follows meetings convened by WHO with its R&D Blueprint technical advisory groups, the Strategic Advisory Group of Experts on Immunization (SAGE), and its Ebola vaccine working group. The experts were reviewing possible vaccines and treatments for Bundibugyo virus disease, which currently has no approved vaccine or cure.

WHO said the goal is to ensure that any use of experimental products generates strong scientific evidence while protecting patients and communities.

For treatment, WHO experts recommended three candidate medicines for evaluation in clinical trials among confirmed cases. These are monoclonal antibodies MBP134 and Maftivimab, and the antiviral drug remdesivir. Experts also suggested studying combination therapy using a monoclonal antibody together with remdesivir.

For prevention, the oral antiviral obeldesivir was identified as a priority candidate for post-exposure use among contacts of infected patients. However, WHO warned that this approach depends on effective contact tracing, which can be difficult in outbreak areas.

On vaccines, WHO identified the single-dose rVSV Bundibugyo vaccine, developed by the International AIDS Vaccine Initiative (IAVI), as one of the most promising candidates. However, it is expected to take 7–9 months before it is ready for full clinical trial evaluation.

Another candidate vaccine, ChAdOx1 Bundibugyo, developed by Oxford University and the Serum Institute of India, could be ready in two to three months for clinical trials, though WHO said more animal data is still required before it can be fully prioritised.

Experts also discussed how different vaccine strategies may be used depending on risk levels. A single-dose vaccine may be used for contacts of confirmed cases, while a two-dose approach could be considered for high-risk groups such as health workers and emergency responders.

Despite Ervebo’s approval for Ebola Zaire, WHO stressed that its effectiveness against Bundibugyo virus remains uncertain and cannot be assumed.

The vaccine “is not licensed for prevention of BVD and evidence on cross-protection to other Ebola virus species remains limited and inconclusive,” WHO said.

WHO also said Ervebo should only be used in research settings when assessing whether it offers any protection against the Bundibugyo strain.

The agency is now working with governments in the DRC and Uganda, the Africa Centres for Disease Control and Prevention (Africa CDC), and other partners to support the design of clinical trials in outbreak areas.

At the same time, WHO warned that vaccines and experimental treatments must not distract from basic outbreak control measures. These include surveillance, rapid testing, isolation of patients, contact tracing, infection prevention in hospitals, and safe and dignified burials.

“In the meantime, our priority is to stop transmission with tools that we have used for decades of Ebola responses, which include disease surveillance, rapid testing and diagnosis, contact tracing, isolation and care for patients, infection prevention and control, community engagement, and safe and dignified burials,” WHO said.

The organisation called for faster access to essential supplies, stronger community trust, and coordinated investment in Ebola research.