There's welcome news in the battle against the Ebola virus, an infectious disease that for years had almost no treatments or remedies.

Outbreaks of the deadly Ebola virus flare up in parts of Africa almost every year, and they're vicious.

"When you see a person who has Ebola, you don't need to be told this is a severe problem," says Oyewale Tomori, a retired virologist from Redeemers University in Nigeria. "They have this ghost-like appearance, bleeding from the orifices. They are weak, they can't move. It's a very devastating experience for those who have it."

The virus kills about half the people it infects. But a new study published in The Lancet Infectious Diseases shows that a promising vaccine (with the complicated name rVSVΔG-ZEBOV-GP) can cut those mortality numbers in half. The results reveal that vaccination doesn't just help to reduce infections — it also reduces deaths from the virus.

"When I first started working in Ebola, we had little more than palliative care to offer patients," says Rebecca Coulborn, an epidemiologist with Epicentre, the medical research arm of Doctors Without Borders. "I think Ebola is a really cruel disease because the very moment when you want to care for someone who you love is the moment when you shouldn't touch them." That's because people are infectious once they develop symptoms.

Over time, however, researchers have developed ways to fight back against Ebola, including rVSVΔG-ZEBOV-GP, a single-dose intramuscular vaccine that causes cells to make one of the virus's proteins. "Later, if the person is exposed to Ebola," explains Coulborn, "their immune system will recognize the viral protein. And this recognition allows the immune system to be prepared to attack the virus and protect the person from Ebola virus disease."

The vaccine is typically administered to those at highest risk of exposure to the virus — a strategy called ring vaccination that targets "people who are contacts of an Ebola case, contacts of contacts and health-care workers," says Coulborn. The vaccine is not yet commercially available.

Researchers showed that rVSVΔG-ZEBOV-GP was highly effective at reducing the risk of infection, but no one knew how capable it was of preventing death in someone who was vaccinated after becoming infected during an epidemic. This is what Coulborn and her colleagues set out to determine.

They focused their efforts on the second-largest Ebola outbreak ever recorded, which took place in the Democratic Republic of Congo between 2018 and 2020. Despite the outbreak flaring up in the midst of an active conflict zone, meticulous records were kept.

"Every single Ebola health facility across the entire Ebola epidemic had a standardized, harmonized and compiled list of all admissions," says Coulborn. This list included 2,279 confirmed Ebola patients, and it recorded whether or not each person had been vaccinated before they got sick — and if so, when they'd received the vaccine. Coulborn then compared how those two groups fared. The result was striking.

Among the unvaccinated, mortality was 56%. But for those who'd received the vaccine, that rate was cut in half. And this was true no matter when someone got vaccinated before the onset of symptoms, whether just a couple days (27.3% fatality risk) or more than three weeks (17.5% fatality risk).

In addition, those who had been vaccinated had less virus circulating in their bodies than those who hadn't. Coulborn says this may help explain the "lower risk of dying, and it could also have an impact on transmission, reducing the spread of Ebola during an epidemic."

"So while getting vaccinated as early as possible is the most beneficial," explains Coulborn, "we now know that vaccination is better late than never."

"This is really exciting news for those of us who are involved in Ebola studies," says Oyewale Tomori, who wasn't involved in the study. During his career, he helped investigate Ebola outbreaks in the Democratic Republic of Congo and Nigeria. He says these results point to how critical vaccination campaigns are during an outbreak, similar to what he and his colleagues have observed with yellow fever.

But Tomori remains curious about just how long this vaccine's protection against Ebola lasts. "What is the duration of that immunity?" he asks. "There's no vaccine that lasts forever."

Rebecca Coulborn says she feels buoyed by the results — since they offer clear evidence that people who are at risk of contracting Ebola should be vaccinated as early as possible. It's an opportunity to cut chains of transmission and hobble an outbreak before it gains speed.

Given how little health workers could do when Ebola first emerged in 1976, Coulborn says the power of this vaccine is remarkable.

"Working in this field has become, I would say, much more hopeful," she says. "Now we can offer people much more than we could in the past."