The only clinical trial data on the experimental Ebola drug ZMapp shows it is 100% effective in monkey studies, even in later stages of the infection.
The researchers, publishing their data in Nature, said it was a "very important step forward".
Yet the limited supplies will not help the 20,000 people predicted to be infected during the outbreak in West Africa.
And two out of seven people given the drug, have later died from the disease.
ZMapp has been dubbed the "secret serum" as it is still in the experimental stages of drug development with, until now, no public data on effectiveness.
Doctors have turned to it as there is no cure for Ebola, which has killed more than 1,500 people since it started in Guinea.
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