The vaccine is designed to spur the immune system's production of anti-Ebola antibodies, and people developed them within four weeks of getting the shots at the National Institutes of Health. Half of the test group received a higher-dose shot, and those people produced more antibodies, said the study published in the New England Journal of Medicine.
Some people also developed a different set of virus-fighting immune cells, named T cells, the study found. That may be important in fending off Ebola, as prior research found that monkeys protected by the vaccine also had that combination response.
Stimulating both types of immune response is "a promising factor," said Dr. Anthony Fauci, director of NIH's National Institute of Allergy and Infectious Diseases, whose employees led the work.
The researchers reported no serious side effects. But two people who received the higher-dose vaccine briefly spiked fevers — one above 103 degrees — which disappeared within a day.
Earlier this month, Fauci told Congress this first-stage testing was promising enough that the U.S. planned much larger studies in West Africa, starting in Liberia in early January, to try to prove whether the vaccine really works.
Scientists are racing to develop ways to prevent or treat the virus that has killed more than 5,600 people in West Africa, most of them in Guinea, Liberia and Sierra Leone.
Wednesday's publication offered scientific details about the initial testing of the vaccine candidate furthest along, one being developed by NIH and GlaxoSmithKline. Additional safety studies are underway here and abroad. A different Canadian-made vaccine also has begun small safety studies.
Many questions remain as larger studies are being designed, including the best dose and how soon protection may begin, cautioned Dr. Daniel Bausch, a Tulane University Ebola specialist who wasn't involved in the study. Plus, monkey research suggests a booster shot will be needed for long-term protection.
"The road is still long and there are many challenges but we are nevertheless one step closer to a solution," he wrote in an accompanying editorial.