Study Shows Patients Vaccinated Against Ebola Are Half as Likely to Die Than Unvaccinated Patients

Results from a study conducted in the Democratic Republic of the Congo (DRC) showed patients with confirmed Ebola virus disease were half as likely to die if they were vaccinated against the virus than if they were not. Results from an initial trial of the vaccine nearly a decade ago during the West African Ebola epidemic indicated it could be 100 percent effective at preventing Ebola, and a subsequent trial conducted by the DRC’s National Institute for Biomedical Research and the World Health Organization during a large outbreak in the DRC determined the vaccine was 97.5 percent effective.

In this new study, researchers analyzed outcomes from 2,279 patients with confirmed Ebola virus disease who were treated in the DRC between July 27, 2018, and April 27, 2020, a study period that coincided with the second largest Ebola outbreak ever recorded, which killed more than 2,200 people. Results showed the fatality rate among 423 participants who had received the vaccine was approximately 25 percent, and among unvaccinated participants, it was 56 percent. What’s more, the vaccine’s protective effect increased the longer a person had been vaccinated, with the mortality rate falling from 27 percent among participants vaccinated just two days or fewer from the onset of their symptoms to 18 percent among those who received the vaccine 10 days or more before they got sick. Most vaccinated participants were aged 15 to 59 years.

The researchers listed several potential reasons other than primary vaccine failure for the more than 400 breakthrough infections identified during the study, including cold chain failure, inadequate dosing or injection technique and immunosuppression.

“Our results reinforce the importance of vaccinating populations who are at risk of exposure to Ebola virus to reduce the risk of infection and — if infection occurs — the risk of death,” the researchers wrote.