The first experimental Ebola vaccine to be tested in humans appears safe and capable of stimulating the immune system, according to a small, early clinical trial at the National Institutes of Health. But the dose of vaccine required to spur the immune system into action is very high, which could make it harder for its manufacturer to quickly produce it, experts say. Still, the results of the 20-person study, launched in September, are promising enough to proceed to a study in West Africa with thousands of participants, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. These trials could take some time, because researchers will have to wait to see if people who are vaccinated contract Ebola. Thomas Geisbert, an Ebola expert at the University of Texas Medical Branch in Galveston, expressed another concern. In animal studies, the NIH vaccine didn't protect for very long, and required a booster to provide long-lasting protection.