LONDON, July 31 (Reuters) -- The world is for the first time on the verge of being able to protect humans against Ebola, the World Health Organization (WHO) said on Friday, as data from a trial in Guinea showed a vaccine was 100-percent effective.
Initial results from the trial, which tested Merck and NewLink Genetics' VSV-ZEBOV vaccine on some 4,000 people who had been in close contact with a confirmed Ebola case, showed 100-percent protection after 10 days.
The results were described as "remarkable" and "game changing" by global health specialists.
"We believe that the world is on the verge of an efficacious Ebola vaccine," WHO vaccine expert Marie Paule Kieny told reporters in a briefing from Geneva.
The vaccine could now be used to help end the worst recorded outbreak of Ebola, which has killed more than 11,200 people in West Africa since it began in December 2013.
WHO Director-General Margaret Chan said the results, published online in the medical journal The Lancet, were an "extremely promising development."
"This is going to be a game changer," she told reporters. "It will change the management of the current Ebola outbreak and future outbreaks."
This and other vaccine trials were fast-tracked with enormous international effort as researchers raced to be able to test potential therapies and vaccines while the virus was still circulating.
"We knew it was a race against time and that the trial had to be implemented under the most challenging circumstances," says John-Arne Røttingen, head of infectious disease control at the Norwegian Institute of Public Health and chair of the trial's steering group.
The Guinea trial began on March 23 to evaluate the effectiveness and safety of a single dose of VSV-ZEBOV using a so-called "ring vaccination" strategy, where close contacts of a person diagnosed with Ebola are immunized -- either immediately, or at a later date.
As data began to emerge showing the very high protection rates in those vaccinated immediately, however, researchers decided on July 26 that they would no longer use the "delayed" strategy, since it was becoming clear that making people wait involved unethical and unnecessary risk.
The trial is now being continued, with all participants receiving the vaccine immediately, and will be extended to include 13- to 17-year-olds and possibly also 6- to 12-year-old children, the WHO said.
Jeremy Farrar, a leading infectious disease specialist and director of the Wellcome Trust, said the trial "dared to use a highly innovative and pragmatic design, which allowed the team in Guinea to assess this vaccine in the middle of an epidemic."
"Our hope is that this vaccine will now help bring this epidemic to an end and be available for the inevitable future Ebola epidemics," his statement said.
The medical charity Doctors without Borders (MSF), which has led the fight against Ebola in West Africa, is now calling for VSV-ZEBOV to be rolled out to the other centers of the outbreak, Liberia and Sierra Leone, where it says it could break chains of transmission and protect front-line health workers.
The success of the Guinea trial is a huge relief for researchers, many of whom had feared that a sharp decline in cases this year would scupper hopes of proving that a vaccine could work.
Another major trial in Liberia, which had aimed to sign up more than 28,000 subjects, had to stop enrolling after only reaching its mid-stage target of 1,500 participants, and plans for testing in Sierra Leone were also scaled back.
That left the study in Guinea, where Ebola is still infecting new victims, as the only real hope for demonstrating the efficacy of a vaccine. (Additional reporting by Tom Miles in Geneva and Ben Hirschler in London. Editing by Kevin Liffey)