MSF study: Ebola vaccine cuts mortality figures in half for people infected with the virus

New findings show vaccination reduces not only the risk of infection, but also the risk of death.

A nurse prepares an Ebola vaccine in Bikoro during the 2018-2020 Ebola epidemic in the Democratic Republic of Congo.

A nurse prepares an Ebola vaccine in Bikoro during the 2018-2020 Ebola epidemic. | Democratic Republic of Congo 2018 © Louise Annaud/MSF

NEW YORK/PARIS, February 9, 2024—A new Doctors Without Borders/Médecins Sans Frontières (MSF) study published in The Lancet Infectious Diseases shows, for the first time, that being vaccinated—even after being exposed to the Ebola virus—can cut mortality figures of those infected in half. The observational study was conducted by MSF’s research arm, Epicentre, and focuses on the rVSVΔG-ZEBOV-GP vaccine—produced by Merck under the name Ervebo—during the 2018-2020 Ebola epidemic in the Democratic Republic of Congo (DRC).

This study found that of the 2,279 confirmed Ebola patients admitted to Ebola health facilities between July 27, 2018 and April 27, 2020, the risk of dying was 56 percent among unvaccinated patients but fell to 25 percent for those who had received the vaccine. This reduction in mortality applied to all patients, regardless of age or gender. MSF’s Epicentre conducted this study with the Institut National de Recherche Biomédicale (INRB) and DRC’s ministry of health.

“Vaccination after exposure to a person infected with Ebola virus disease—even when administered shortly before the onset of symptoms—still confers significant protection against death," said Rebecca Coulborn, an epidemiologist with MSF’s Epicentre.

rVSVΔG-ZEBOV-GP, which is effective against the especially deadly Zaire strain of Ebola, is the only Ebola vaccine recommended for use during an epidemic like the one in DRC in 2018-2020. Designed to be administered in a single dose, it is recommended primarily for ring vaccination. Ring vaccination is a strategy that targets contacts and contacts of contacts of confirmed Ebola cases, as well as health care and frontline workers who are at high risk of exposure. While the goal is to vaccinate people as early as possible during outbreaks, these new findings show that vaccinating someone who has already been exposed but is not yet symptomatic can also reduce the risk of death.

Prior to this Lancet study, a previous phase 3 clinical trial conducted in Guinea found that the rVSVΔG-ZEBOV-GP vaccine greatly protects against Ebola virus disease. However, some people, despite being vaccinated for more than 10 days—the period considered sufficient to develop immunity—were still infected with the virus during the 2018-2020 Ebola epidemic in the DRC. The mortality of those who were vaccinated and still fell ill had not been evaluated prior to this Lancet study. These new findings help make a case for the use of a combination of vaccination and treatment after someone is exposed to the virus.

“In addition to the direct benefit, our results allow us to consider combining vaccination and treatment of patients who have been in direct contact with a person with confirmed Ebola virus disease in order to reduce the risk of illness and death,” said Etienne Gignoux, director of the Epidemiology and Training Department at MSF’s Epicentre.