An experimental Ebola vaccine partially designed at Canada's National Microbiology Laboratory was found to be highly protective in a major clinical trial in Guinea, researchers report, suggesting future outbreaks could be easily contained.

The largest Ebola outbreak in history swept through West Africa in 2014 and 2015, resulting in more than 15,000 lab-confirmed cases and more than 11,300 deaths.

The mortality rate during the epidemic ranged between 50 to 80 per cent.

The clinical trial involved the rVSV-ZEBOV vaccine and took place in Guinea's coastal region of Basse-Guinée, which was still seeing new Ebola cases when the experiment launched in 2015.

The results were published Thursday in the medical journal The Lancet.

Among the 5,837 people who received the vaccine, no Ebola cases were recorded 10 days or more after vaccination, signalling a 100 per cent protection rate.

To conduct the trial, the researchers used the ring strategy, which was the same approach used to help eradicate smallpox.

In ring vaccination, an infected person is first identified. Then "rings" or clusters of anyone who came in contact with the person are vaccinated. Finally, friends, family and neighbours of those contacts are vaccinated.

At first, the clusters were assigned to receive the vaccine either immediately or after a three-week delay, with those in the delayed group acting as a control. After the preliminary results were encouraging, researchers changed the design of the trial to allow everyone to be given a shot immediately.

In the groups that were immediately vaccinated, no cases of Ebola occurred 10 days or more after vaccination. In comparison, the control groups that received a delayed vaccination saw 23 cases of infection.

Marie-Paule Kieny, assistant director-general of health systems and innovation at the World Health Organization, said it was a joy to see the first results on the efficacy of an Ebola vaccine. (Martial Trezzini/Keystone/AP)

"This vaccine seems to be highly efficacious in preventing Ebola in people at risk," lead author Marie-Paule Kieny said in an interview. "It was such a joy when we saw the first results."

There are currently no formal recommendations on when to use the experimental Ebola vaccine. But Kieny, who is WHO's assistant director-general for health systems and innovation, imagines two likely scenarios:

Immediately, in a reactive manner, if an outbreak is identified.

Preventively to protect health-care workers or front-line workers in countries vulnerable to Ebola.

Gary Kobinger worked on the initial vaccine at the National Microbiology Laboratory in Winnipeg and spent seven months in West Africa at the height of the Ebola epidemic. He calls the findings "phenomenal news."

"There should never be another outbreak like the one in West Africa," said Kobinger. "This vaccine is one tool that [many researchers] are now very much hoping will be available and will contribute to controlling the next outbreak much faster and much more efficiently."

Despite the challenges of running a clinical trial during an epidemic, the researchers say the study shows it's feasible.

How long-lasting is the vaccine?

The vaccine still needs to be approved by regulatory authorities, such as the U.S. Food and Drug Administration, and be commercially produced. But it would be reasonable for countries in West Africa to store some doses that could be dispatched in the case of an outbreak, Kobinger said.

At the time of the Ebola outbreak, no vaccine was available, spurring global development efforts.

There should never be another outbreak like the one in West Africa. - Ebola researcher Gary Kobinger

"Protective efficacy is clearly the strength of the study," Thomas Geisbert, a microbiology professor at the University of Texas Medical Branch, wrote in a journal commentary published alongside the study.

While the vaccine seems safe in the context of an outbreak, Geisbert said he's left with some questions. For example, is the vaccine long-lasting? Or is it still protective two or three years after the vaccination?

Durability of the vaccine should be a key consideration when enrolling lab workers to receive the vaccine, Kobinger said.

"I think, at this point, to have an exposure leading to a fatality would be more than sad; it would be unacceptable now that this vaccine is available."

The vaccine was developed by the Public Health Agency of Canada before being licensed to NewLink Genetics, who in turn licensed it to Merck & Co.

The study was funded by WHO, UK Wellcome Trust, Médecins Sans Frontiè​res, the Norwegian Ministry of Foreign Affairs and the Canadian government.