"Everybody's been racing to get trials launched as quickly as we could and as carefully as we could," said Dr. Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases.

In addition to the vaccines, Fauci said scientists also will test the Ebola drug ZMapp in the US and Liberia. The experimental treatment made headlines when it was given to several Ebola patients last summer, before manufacturer Mapp Biopharmaceutical Inc. ran out of doses. But it has not been formally tested for safety and effectiveness in people, and US officials said there now is enough supply to begin doing so.

The World Health Organization says the Ebola epidemic has infected more than 21,000 people and claimed more than 8,600 lives. Without a vaccine, officials have fought the outbreak with old-fashioned public health measures, including isolating the sick, tracking and quarantining those who had contact with them, and setting up teams to safely bury bodies.

Fauci said both experimental vaccines showed promise in first-stage human safety tests. One was developed by the NIH and is being manufactured by GlaxoSmithKline. The other was developed by Canadian health officials and is licensed to two US companies, NewLink Genetics and Merck. Each uses a different virus to carry non-infectious Ebola genetic material into the body and spark an immune response.

The study in Liberia will randomly assign people to get either one of the vaccines or a dummy shot, considered the best way to prove if there's really an effect. Fauci said even if the current outbreak continues to wane - "which would be a good thing for all" - that it would be possible to get some idea of how effective the vaccines are. Some of the participants will undergo special testing to see how their immune systems respond to the shots.