Merck and NewLink’s experimental Ebola vaccine rVSV-ZEBOV has shown 100% effectiveness in a trial in West Africa.

The study took place in Guinea, one of the countries most affected by the current outbreak, and used a ‘ring design’. Rather than use a placebo group, these trials found people who had been in contact with someone with Ebola and vaccinated half of them immediately, and the other half three weeks later.

In the groups that were vaccinated immediately, there were no cases of Ebola from 10 days after vaccination – the time allowed for the immunity to develop. In the delayed-vaccination group, there were 16 cases, a statistically significant difference.

Børge Brende, the foreign minister of Norway, which helped fund the trial, called the result “sensational” and said rVSV-ZEBOV may be a “silver bullet against Ebola, helping to bring the current outbreak to zero and to control future outbreaks of this kind”.

These results set Merck’s vaccine as the most promising of all the candidates currently being investigated, which include offerings from Janssen and GlaxoSmithKline.

rVSV-ZEBOV was originally developed by the Public Health Agency of Canada and licensed to NewLink in 2010, which subsequently brought Merck on board in a licensing and collaboration deal last year.