Twenty volunteers were immunized in the United States and none suffered major side effects. All produced antibodies according to research published Wednesday the New England Journal of Medicine.

"The unprecedented scale of the current Ebola outbreak in West Africa has intensified efforts to develop safe and effective vaccines," said Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases (NIAID), which is developing the vaccine alongside GlaxoSmithKline.

The vaccines under development "may play a role in bringing this epidemic to an end and undoubtedly will be critically important in preventing future large outbreaks," he said.

The news comes amid the worst ever outbreak of the hemorrhagic fever, which has killed nearly 5,700 people in West Africa, most of them in Guinea, Liberia and Sierra Leone.

In the first phase of testing, the 20 healthy adults were divided into two groups, receiving a higher or lower dose of the vaccine. The volunteers were injected starting in September.

The researchers reported no serious side effects but two people who received the higher-dose vaccine briefly spiked fevers, one above 103 degrees Fahrenheit, which disappeared within a day.

Each of the volunteers showed a positive result for Ebola antibodies in blood tests within four weeks, according to the National Institutes of Health (NIH), which conducted the study.

The 10 volunteers in the higher-dose group developed higher antibody levels, the NIH said.

In addition, two of the lower-dose group and seven of the higher-dose group developed a kind of immune cell called CD8 T cells, which are an important part of the body's response against disease.

"We know from previous studies in non-human primates that CD8 T cells played a crucial role in protecting animals" who got the vaccine and then were exposed to Ebola, said researcher Julie Ledgerwood, the trial's principal investigator.

The NIAID is "in active discussions with Liberian officials and other partners about next-stage vaccine testing in West Africa" for efficacy and safety, the NIH said, but no announcement on larger-scale trials was expected before early next year.

Another experimental vaccine that has shown promising results in primates is the Canadian VSV-EBOV, licensed by the U.S.-firm NewLink Genetics. It is also in early stages of human testing.

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.

There is no licensed treatment or vaccine against the Ebola virus, which is transmitted through bodily fluids and has been fatal in an estimated 70 percent of cases in the current outbreak.

The World Health Organization said Thursday that the global death toll from the Ebola virus had increased to 5,689 out of a total of 15,935 cases of infection, almost entirely in western Africa.

The WHO believes that the number of deaths is likely far higher, given the difficulty in collecting comprehensive figures and Ebola's high fatality rate.

Al Jazeera and wire services

