“This is a landmark study,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, who wasn’t involved in the trial.

The vaccine took effect within 10 days of vaccination, according to the study, published in The Lancet. that could make the vaccine very useful in an outbreak, Osterholm said.

“This is an extremely promising development,” said Margaret Chan, Director-General of the WHO. “The credit goes to the Guinean government, the people living in the communities and our partners in this project. An effective vaccine will be another very important tool for both current and future Ebola outbreaks.”