Researchers say an experimental vaccine has shown to be "highly effective" at preventing the deadly disease Ebola.

The World Health Organization said Friday that the vaccine has been 100 percent effective at tests performed in Guinea. The British medical journal Lancet has published early test results and analyses of the findings.

Norway's foreign minister praised the effectiveness of the new vaccine, calling it "the silver bullet" against Ebola. Norway is one of the several countries involved in the testing.

Ebola spread through West Africa last year. The disease killed more than 11,000 people -- most of them in Liberia, Guinea and Sierra Leone.

Ebola is one of a number of highly dangerous viruses known as hemorrhagic fevers. They can affect many organs, damage blood vessels and affect the body's ability to fight infection. There are hopes the new vaccine may be a success in fighting against all such diseases.

The vaccine is known as VSV-EBOC. Beginning in March, it was tested on more than 4,000 people in Guinea. All of them had come into close contact with the Ebola virus.

WHO officials say early results were so promising that random tests were stopped this week so everyone at risk could receive the vaccine immediately.

Vaccine could be ‘game-changer'

The WHO's general director, Margaret Chan, spoke to reporters in Geneva. She said that scientists, doctors and drug companies worked together in development of the vaccine.

If proven effective, she said, the vaccine "is going to be a game changer." She said it will affect efforts to control the current Ebola outbreak and future outbreaks.

Researchers decided to change their efforts against the disease as the testing began. One reason was a sharp drop in the number of new cases in Africa over the past few months. The other was the changing nature of the Ebola epidemic.

Last year, the disease spread very quickly over large parts of West Africa. But then it seemed to change, resulting in a series of sudden, small outbreaks across the area.

The ideal way for testing a new drug would be to vaccinate half of a population at risk of contracting Ebola. At the same time, other people in the virus area would receive a placebo or harmless substance.

Instead the team working on the vaccine chose to use a "ring" design. When a first Ebola case was reported in a village, researchers vaccinated everyone in contact with the sick person. Such "rings" of vaccination took place either immediately after an Ebola infection was confirmed or three weeks later.

Of the 2,014 people vaccinated immediately, no one had Ebola when tested 10 days after receiving the new drug. A total of 2,380 people were vaccinated three weeks later. Only 16 of them became infected with the virus.

Test results will now go to government regulatory agencies. The agencies will decide whether to approve a license for the vaccine. Such a permit would enable production of the vaccine for future Ebola epidemics.

There have been a total of 27,748 cases of Ebola in Guinea, Liberia and Sierra Leone. The three countries have at least 11,279 Ebola deaths. However, the result of many cases is not yet known, and the real number of death will be higher.

I'm Anna Matteo.

Lisa Schlein reported on this story from Geneva. Triwik Kurniasari adapted it for Learning English. George Grow was the editor.

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analyses – n. commentaries on a report or study

journal – n. a publication

outbreak – n. the sudden increase of disease

random – adj. chosen without plan; not targeted

contract(ing) – v. becoming sick with disease

license – n. an official document that gives you permission to do something; a permit

epidemic – n. an infectious disease moving across a wide area

game changer – n. someone or something with a very powerful influence