Two experimental Ebola vaccines have shown promise in protecting against the virus for at least a year, according to the results of a major clinical trial.

The report, in the New England Journal of Medicine, is the first full account of a large-scale effort to test what could be the first vaccine against Ebola.

The outbreak of the highly contagious and often deadly virus killed more than 11,000 people, mainly in Liberia, Guinea and Sierra Leone, when it swept west Africa from late 2013 to 2016.

The phase II study involved 1,500 people in Monrovia, Liberia.

Participants were randomly assigned to receive one of two vaccines being tested, or a placebo.

The vaccine candidates included the frontrunner, rVSV-ZEBOV, initially engineered by Canadian government scientists and now licensed to Merck, Sharp and Dohme Corporation.

The second vaccine candidate was cAd3-EBOZ, co-developed by the US National Institute for Allergy and Infectious Diseases' (NIAID) Vaccine Research Center and GlaxoSmithKline.

After one month, 84% of rVSV-ZEBOV recipients developed an antibody response. At one year, 80% still had this protection.

For the other candidate, cAd3-EBOZ, 71% developed an antibody response, and 64% still had such a response at one year, when the trial ended.

Some people receiving the vaccines experienced "mild to moderate side effects that resolved, such as headache, muscle pain, feverishness and fatigue," said the report.

Overall, investigators did not identify any major safety concerns related to the vaccines.

The trial was conducted as part of a US-Liberia clinical research collaboration known as the Partnership for Research on Ebola Virus in Liberia (PREVAIL).

As many as 15 vaccine candidates are being designed worldwide to prevent Ebola.

Experts say the first vaccines could be approved for use by 2018, under a fast-track regulatory process.