An experimental Ebola vaccine initially developed by the Public Health Agency of Canada has proven to be “highly protective” against the deadly virus in a major trial in Guinea, according to results published on December 23 in The Lancet. The vaccine is the first to prevent infection from one of the most lethal known pathogens.

The vaccine was studied in a trial involving 11,841 people in Guinea. Among the 5,837 people who received the vaccine, no Ebola cases were recorded 10 days or more after vaccination. In comparison, there were 23 cases 10 days or more after vaccination among those who did not receive the vaccine.

The trial was led by the World Health Organization (WHO), together with Guinea’s Ministry of Health, Medecins sans Frontieres and the Norwegian Institute of Public Health, in collaboration with other international partners.

The trial used an innovative design, a so-called “ring vaccination” approach – the same method used to eradicate small pox. When a new Ebola case was diagnosed, the research team traced all people who may have been in contact with that case within the previous three weeks, such as people who lived in the same household, were visited by the patient, or were in close contact with the patient, their clothes or linen, as well as certain “contacts of contacts.” A total of 117 clusters (or “rings”) were identified, each made up of an average of 80 people.

Initially, rings were randomized to receive the vaccine either immediately or after a three-week delay, and only adults over 18 years were offered the vaccine. After interim results were published showing the vaccine’s efficacy, all rings were offered the vaccine immediately, and the trial was also opened to children older than six years.

In addition to showing high efficacy among those vaccinated, the trial also shows that unvaccinated people in the rings were indirectly protected from Ebola virus through the ring vaccination approach (so called “herd immunity”). However, the authors note that the trial was not designed to measure this effect, so more research will be needed.

In January, GAVI, the Vaccine Alliance provided $5 million to the manufacturer of the vaccine, Merck, towards the future procurement of the vaccine once it is approved, prequalified and recommended by WHO. As part of this agreement, Merck committed to ensure that 300,000 doses of the vaccine are available for emergency use in the interim, and to submit the vaccine for licensure by the end of 2017. Merck has also submitted the vaccine to WHO’s Emergency Use and Assessment Listing procedure, a mechanism through which experimental vaccines, medicines and diagnostics can be made available for use prior to formal licensure.