An experimental Ebola vaccine tested on humans was highly protective against the deadly virus in a major trial in Guinea, a major milestone that could help prevent the spread of outbreaks like the one that killed thousands in West Africa, the World Health Organisation, WHO, announced today.

The vaccine is the first to prevent infection from one of the most lethal known pathogens, and the findings, add weight to early trial results published last year.

Results, published in the medical journal The Lancet, show that of nearly 6,000 people receiving the vaccine, all were free of the virus 10 days later The vaccine, called rVSV-ZEBOV, was studied in a trial involving 11 841 people in Guinea during 2015. Among the 5837 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 WHO, together with Guinea’s Ministry of Health, Medecins sans Frontieres and the Norwegian Institute of Public Health, in collaboration with other international partners.

"While these compelling results come too late for those who lost their lives during West Africa's Ebola epidemic, they show that when the next Ebola outbreak hits, we will not be defenceless," said Dr Marie-Paule Kieny, WHO’s Assistant Director-General for Health Systems and Innovation, and the study’s lead author, in a statement published on the WHO's website.

The vaccine's manufacturer, Merck, Sharpe & Dohme, this year received Breakthrough Therapy Designation from the United States Food and Drug Administration and PRIME status from the European Medicines Agency, enabling faster regulatory review of the vaccine once it is submitted.

Since Ebola virus was first identified in 1976, sporadic outbreaks have been reported in Africa. But the 2013–2016 West African Ebola outbreak, which resulted in more than 11 300 deaths, highlighted the need for a vaccine.

The trial took place in the coastal region of Basse-Guinee, the area of Guinea still experiencing new Ebola cases when the trial started in 2015. 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 3 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 randomised to receive the vaccine either immediately or after a 3-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 6 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.