Today, The Lancet released the results of a large field trial of a vaccine against Ebola, and the results are more than promising. Within the limitations of the study, the vaccine appears to be 100 percent effective. The results were so good that the trial itself has been stopped, and the vaccine is now being used to control the spread of the disease.

The vaccine is made by the pharmaceutical giant Merck, which licensed it from the Public Health Agency of Canada. It was developed through what has become a fairly standard approach. A harmless virus (vesicular stomatitis virus, or VSV) was engineered so that it also carried the gene for Ebola's major surface protein, simply called glycoprotein. When people receive the vaccination, a harmless infection follows, which triggers an immune response. This response targets not only VSV but the Ebola protein as well. Ideally, once the infection is eliminated, the immune system is able to recognize both VSV and Ebola.

The trial, performed in southern Guinea, ran from April through July 20th of this year (the analysis, paper writing, and peer review must have proceeded at a staggering pace). It used what is called a "ring" design: once an infected individual was identified, a ring of potentially exposed individuals around them was identified. These individuals lived with the infected one, had contact with them after symptoms appeared, or came in contact with their clothes, bedding, or bodily fluids.

Over the course of the study, 90 rings were identified, with a total of 7,651 people in them. These individuals were then randomized to either receive an immediate vaccination or a vaccination 21 days later. Six follow-up visits over the next few weeks tracked whether the individuals were infected and looked for any adverse effects from the vaccine itself.

Within 10 days of the first vaccinations, an effect was obvious. Sixteen confirmed cases were found among those in the delayed vaccination group; none were found in those who were given immediate vaccination. Eventually, 75 cases in total were identified. Six of these occurred in the immediate vaccination group, but in every single one of these, the individuals had never received the vaccine, either because they didn't provide informed consent or due to logistical issues.

The vaccine also appeared to be safe. Although 43 serious adverse events occurred, most of them appeared to be unrelated to the vaccine ("adverse events" included things like car accidents). The only one that appeared to be vaccine-related was a single individual that developed a fever that cleared up on its own. There was also a slight indication that herd immunity occurred, in which vaccinated individuals block the further spread of the virus, but the trial wasn't large enough for this result to reach statistical significance.

While all of that is staggeringly promising, perhaps the most significant finding was that the vaccination program could be rolled out in what the authors term "resource-poor settings and in a crisis situation." Guinea's national coordinator for the Ebola response stated, "This is Guinea’s gift to West Africa and the world. The thousands of volunteers from Conakry and other areas of Lower Guinea, but also the many Guinean doctors, data managers and community mobilizers have contributed to finding a line of defence against a terrible disease."

The basic design of the trial—rapid case identification and ring vaccination—can also be very effective in eliminating outbreaks (it has been used against smallpox in the past). So the trial itself provides a model for future Ebola control efforts, and the experienced gained through it could be invaluable. It's notable that only half of those who were chosen for immediate vaccination actually managed to receive it, so there's still some work to do. But it's hard to argue with Dr Margaret Chan, director-general of the World Health Organization, who said, "This is an extremely promising development."

The Lancet, 2015. DOI: 10.1016/S0140-6736(15)61117-5 (About DOIs).