US officials say a long-awaited major study of possible Ebola vaccines is set to begin in Liberia within a couple of weeks.
The study will compare two vaccine candidates with dummy shots to test both safety and whether either of the shots really protects against the Ebola virus.
The virus has devastated Liberia, Guinea and Sierra Leone over the past year.
New infections are falling, but Dr Anthony Fauci of the National Institutes of Health said it's not too late for the vaccine study.
Clusters of cases continue, and he said some participants would undergo special testing to see how their immune systems respond to the vaccine.
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Fauci said up to 27,000 people may ultimately be enrolled.
A smaller study also is being planned for Sierra Leone.
