Dr. Margaret Chan

Dr. Margaret Chan told the U.N. Security Council on Thursday that there have been no new cases in Liberia, and only three new cases in Sierra Leone and Guinea, in each of the last two weeks, the lowest numbers in well over a year.

She says fears that the Ebola virus could be permanently established in humans in the region have also been defeated, which is "very good news."

But Chan also cautions against "a false sense of security."

There is currently no licensed treatment or vaccine for Ebola, which has so far killed more than 11,000 people in West Africa since the world's biggest outbreak began in the forest region of Guinea last year.

"If proven effective, this is going to be a game-changer," said Dr. Margaret Chan, Director-General of the World Health Organization, which sponsored the study. "It will change the management of the current outbreak and future outbreaks."