An experimental Ebola vaccine made by GlaxoSmithKline caused no serious side effects and produced an immune response in all 20 healthy volunteers who received it in an early-stage clinical trial, scientists reported on Wednesday in the New England Journal of Medicine.

The trial, which began on Sept. 2 and will monitor the volunteers for 48 weeks, is primarily aimed at assessing how safe the vaccine is. But the immune response offered hope that it would also be effective.

"The safety profile is encouraging, as is the finding that the higher dose of vaccine induced an immune response quite comparable to that which has completely protected (lab) animals from Ebola,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), which is conducting the trial in Bethesda, Maryland.

The intramuscular vaccine was developed at NIAID and Okairos, a biotechnology company acquired by GlaxoSmithKline. It contains genetic material from two Ebola strains - Zaire, responsible for the current outbreak in West Africa, and Sudan - but no virus, so it cannot cause the disease.

Because it is unethical to expose volunteers to Ebola, researchers assess the effectiveness of candidate vaccines by whether they trigger production of anti-Ebola antibodies and immune-system T cells.

The trial enrolled volunteers ages 18 to 50. Half received a lower dose and half a higher dose. All 20 developed anti-Ebola antibodies within four weeks, with those on the higher dose producing more.

Dose also affected production of T cells; seven of 10 people on the high dose produced one crucial kind of T cell, but only two on the low dose did. The higher the dose required to trigger immunity, the more challenging and expensive it will be to produce large quantities of vaccine, manufacturers say.

Dr Daniel Bausch of Tulane University, who wrote an accompanying commentary, called the results promising but cautioned that there are many more challenges ahead before the vaccine’s safety and efficacy are established.

Another GlaxoSmithKline vaccine, against the Zaire strain, is undergoing safety trials in England, Mali and Switzerland, while one from Iowa-based NewLink Genetics is being tested in Maryland. This week, Merck announced that it would buy the rights to NewLink’s vaccine for $50 million. A trial of an Ebola vaccine from Johnson & Johnson is scheduled to start in January.