There is currently no licensed treatment or vaccine for Ebola, which has so far killed more than 11,000 people in West Africa since the world's biggest outbreak began in the forest region of Guinea last year.
"If proven effective, this is going to be a game-changer," said Dr. Margaret Chan, Director-General of the World Health Organization, which sponsored the study. "It will change the management of the current outbreak and future outbreaks."
Scientists have struggled for years to develop Ebola treatments and vaccines but have faced numerous hurdles, including the sporadic nature of outbreaks. Many past attempts have failed, including a recently abandoned drug cocktail being tested in West Africa by Tekmira Pharmaceuticals.
Researchers gave one dose of the new vaccine, developed by the Canadian government, to more than 4,000 people who were contacts of confirmed Ebola cases within 10 days of being identified. In comparison, more than 3,500 contacts of other Ebola cases got the shot after a 10-day delay. In the group that received the vaccine immediately, there were no Ebola cases versus 16 cases in people who got delayed vaccination. The vaccine has since been licensed to Merck. The study was published online Friday in the journal Lancet.
An expert group monitoring the study's data and safety recommended the trial be stopped on July 26 so that everyone exposed to Ebola in Guinea could get immunized.
The vaccine uses an Ebola protein to prompt the body's immune system to attack the virus.
"It looks to be about as safe as a flu vaccine," said Ben Neuman, a virologist at the University of Reading who was not part of the trial. Researchers are still assessing possible side effects; the most serious seemed to be fever and the stress experienced by patients who believe such symptoms were due to Ebola.
"This (vaccine) could be the key that we've been missing to end the outbreak," Neuman said. "I don't see any reason on humanitarian grounds why it should not be used immediately." He said further tests would be necessary to see if the vaccine might also protect pregnant women, children and adolescents; those trials are already under way.
WHO vaccines expert Marie-Paule Kieny said having an effective immunization might avert future disasters but added it would still take months to get the shot licensed.
"Using a tool like this vaccine, we would be able to stop the epidemic from going really wild and spreading further," she told reporters, noting that stamping out future outbreaks still depends on early detection. WHO first identified Ebola in Guinea last March but did not declare the epidemic to be a global emergency until August, when the virus had killed nearly 1,000 people.
Other Ebola vaccines are being studied elsewhere but the declining caseload — there were just seven new patients reported in West Africa last week — is complicating efforts to finish the trials.
Dr. Bertrand Draguez of Doctors Without Borders, which helped test the vaccine in its treatment clinics in Guinea, said the immunization should immediately be made available.
"With such high efficacy, all affected countries should immediately expand vaccination of contacts of infected patients in order to break chains of transmission and vaccinate all front-line workers to protect them," he said in a statement.