Mankind took one step closer to an Ebola vaccine, as one candidate passed initial tests with flying colors.

Tests determined that an Ebola vaccine candidate was safe and prompted the immune systems in test subjects to develop antibodies to fight the disease. The results were published in the New England Journal of Medicine, and the research was spearheaded by the National Institute of Allergy and Infectious Diseases and GlaxoSmithKline, according to the Los Angeles Times.

To test the vaccine, researchers put genetic material from Ebola in 20 adults at NIAID’s Vaccine Research Center. It was not the actual Ebola virus, so the test subjects were not in danger of contracting the disease, although two participants came down with a fever a day after the vaccination.

The vaccine uses a genetically modified chimpanzee cold virus. It is one of several candidates to stop the disease, which has ravaged West Africa and killed more than 5,600 to date, with 15,900 sickened.

The 20 people were split into two groups, and one group was given a larger dose than the other. The larger dose provoked a bigger response from the immune system, although it caused fever in two subjects.

Scientists knew the vaccine was effective for monkeys, but didn’t know what dosage it would take for humans.

Study authors cautioned against becoming two optimistic about the results, noting that it was only a trial and much work needed to be done. However, the fact that there were no adverse effects were encouraging, and researchers will do more work to ensure that they get the dosage right.

The vaccine will attempt to fight two strains of Ebola: Zaire and Sudan, with the former being the form of virus that is causing the current outbreak.