A newly developed vaccine against the deadly Ebola virus is “highly effective” and could help prevent its spread in the current and future outbreaks, the World Health Organization said Friday. Trials of the single-dose VSV-EBOV vaccine began in March in Guinea — one of three West African nations at the center of the outbreak — and have shown such promise that this week it was decided to extend immediate vaccination to “all people at risk” after close contact with an infected person, a WHO news release said.