Ebola Resource Center

Ebola Resource Center

Issue: November 2017
October 16, 2017
3 min read

Ebola vaccines elicit yearlong immune response in Liberia trial

Issue: November 2017
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Anthony Fauci
Anthony S. Fauci

Two candidate Ebola virus vaccines elicited immune responses that were mostly maintained through 1 year in a large clinical trial in Liberia, according to findings published in The New England Journal of Medicine.

The trial, which took place during the 2014-2016 West African Ebola epidemic, evaluated the performance of two vaccines developed in the United States and Canada.

One of the two vaccines — rVSV∆G-ZEBOV-GP (Merck) — was 100% effective at preventing Ebola in an earlier trial in Guinea. The vaccine was also used during a flare-up of Ebola in Guinea last year — the first time an Ebola vaccine has ever been used in an outbreak setting outside a clinical trial. The vaccine was approved for use during a recent outbreak in the Democratic Republic of the Congo but was not needed.

In a statement, National Institute of Allergy and Infectious Diseases Director Anthony S. Fauci, MD, said the trial “yielded valuable information that is essential for the continued development of these two Ebola vaccine candidates and also demonstrates that well-designed, ethically sound clinical research can be conducted during an epidemic.”

“A safe and effective vaccine would be a critically important addition to classical public health measures in controlling inevitable future Ebola outbreaks,” Fauci said.

PREVAIL findings

Researchers from the Partnership for Research on Ebola Virus in Liberia, or PREVAIL, a clinical research partnership between the U.S. and Liberia, conducted the trial. As cases declined and the epidemic ended, researchers scrapped plans for an intended phase 3 trial and reported results of an embedded phase 2 sub-trial that evaluated the safety and immunogenicity of the two vaccines, ChAd3-EBO-Z (GlaxoSmithKline), which the NIAID co-developed, and rVSV∆G-ZEBOV-GP, which the Public Health Agency of Canada initially engineered.

For almost 3 months in 2015, researchers enrolled 1,500 volunteers aged 18 years and older with no reported history of Ebola virus disease at Redemption Hospital in Monrovia to receive one of the two vaccines or placebo. They drew blood samples from the study participants at vaccination, then again at 1 week, 1 month, 6 months and 1 year after vaccination.

By 1 month, an antibody response developed in 70.8% of participants vaccinated with ChAd3-EBO-Z and 83.7% in the rVSV∆G-ZEBOV-GP group compared with 2.8% in the placebo group, the researchers reported. At 1 year, these antibody responses were 63.5% in the ChAd3-EBO-Z group, 79.5% in the rVSV∆G-ZEBOV-GP group, and 6.8% in the placebo group.

The researchers did not identify any safety concerns related to the vaccines. The percentage of participants who developed malaria was lower in each of the vaccine groups compared with the placebo group — an “unexpected finding” suggesting cross-reactivity, they said, although further study is needed to determine if it is possibly a result of trained immunity or a chance finding.

“In Liberia, we have demonstrated to the global community that rigorous scientific research can take place in a developing sub-Saharan African country when a mutually beneficial partnership is developed,” Liberian Minister of Health Bernice T. Dahn, MD, MPH, said in a statement. “The work of PREVAIL, ranging from the Ebola vaccine to the Ebola survivor studies, clearly manifest the prospects of such a sustainable partnership and clinical research platform.”

Trial adds pediatric data

Last year, researchers from the Ebola Ça Suffit! (Ebola That’s Enough!) trial published data showing the effectiveness of rVSV∆G-ZEBOV-GP during a ring vaccination trial in 2015 in an area of Guinea that was still experiencing new infections — the first vaccine to show protection against Ebola. During a flare-up of Ebola in March 2016, researchers from the same trial traveled to the site of the flare-up and performed another test of the vaccine.

Between March 17 and April 21, 2016, they vaccinated 1,510 contacts, and contacts of contacts, of Ebola patients in four rings, including 303 adolescents aged between 6 and 17 years and 307 front-line workers, and followed them for 21 days.

According to the findings, no secondary cases of Ebola occurred among those who received vaccination and there were no reports of severe vaccine-related adverse events. The researchers said the results showed that a ring vaccination strategy “can be rapidly and safely implemented at scale in response” to Ebola outbreaks in rural settings.

“This article adds substantially to previously published results from the Ebola Ça Suffit! ring vaccination phase 3 trial of the same vaccine, including much needed pediatric data,” John S. Schieffelin, MD, assistant professor of medicine and pediatrics at Tulane University School of Medicine, wrote in an accompanying editorial. – by Gerard Gallagher


Gsell PS, et al. Lancet Infect Dis. 2017;doi:10.1016/S1473-3099(17)30575-3.

Kennedy SB, et al. N Engl J Med. 2017;doi:10.1056/NEJMoa1614067.

Schieffelin JS, et al. Lancet Infect Dis. 2017;doi:10.1016/S1473-3099(17)30575-3.

Disclosures: Dahn, Fauci and Schieffelin report no relevant financial disclosures. Please see the studies for all authors relevant financial disclosures.