The new vaccine has some flaws, experts said. It appears to work only against one of the two most common strains of the Ebola virus, and it may not give long-lasting protection. Some of those who get it report side effects like joint pain and headaches.

“It’s certainly good news with regard to any new outbreak — and one will occur somewhere,” said Dr. Anthony S. Fauci, director of the National Institute for Allergy and Infectious Diseases, which makes many vaccines and did some early testing on this one. “But we still need to continue working on Ebola vaccines.”

The Lancet study was done in 11,841 residents of Guinea last year. Among the 5,837 people who got the vaccine, none came down with Ebola 10 or more days later. There were 23 Ebola cases among the thousands of others not immediately vaccinated.

(The 10-day window was important because the trial used the “ring vaccination” technique developed during the drive to eliminate smallpox. Once a confirmed case was found, researchers contacted everyone in the circle of family, friends, neighbors and caregivers around the victim. About half the “circles” were offered vaccine. No one who fell ill within the first nine days after vaccination was counted, however, because it was assumed that they had already been infected before vaccination.)

The Ebola trial was led by the World Health Organization, the Guinean Health Ministry, Norway’s Institute of Public Health and other institutions. The vaccine, known as rVSV-ZEBOV, was developed over a decade ago by the Public Health Agency of Canada and the United States Army and is now licensed to Merck.

Its genetic “spine” is that of a vesicular stomatitis virus, which sickens cattle but usually does not infect humans. Spliced into the spine is the gene coding for an Ebola virus surface protein that prompts the immune system to make antibodies.