The government is speeding up its development of several potential Ebola vaccines in response to the largest ever outbreak of the virus in West Africa.
The National Institutes of Health (NIH) confirmed Thursday that it will start testing a vaccine candidate on humans next week for the first time ever.
Despite the progress, the NIH said it could not predict when vaccines might be available to use against the current epidemic.
"It's really impossible to predict when people will be able to have it to use," said Anthony S. Fauci, director of the NIH's National Institute of Allergy and Infectious Diseases.
"We will get data on safety and whether this [initial] vaccine induces the kind of immune response we hope to see ... by the end of this calendar year. Then, we have to look at what those results are."
Fauci and NIH Director Francis Collins tamped down questions about whether a vaccine could make a difference on the ground in Africa this year.
"The real solution is to implement public health measures" to contain the virus, Fauci said.
The first vaccine candidate was developed by the National Institute of Allergy and Infectious Diseases and leading drugmaker GlaxoSmithKline.
The first three volunteers in the trial will receive the vaccine next week at an NIH facility in Bethesda, Md.
The announcement comes after significant pressure on U.S. officials to respond to the rising number of Ebola deaths with experimental vaccines and treatments. The NIH did not announce any human trials for Ebola drugs, saying those medications remain in the pre-clinical trial stage, despite some limited use in Americans who contracted the virus in West Africa.
Fauci said officials must be cautious as they develop the Ebola vaccines and any potential treatments.
"This is the first time the vaccine has been in a human," he said of the new trial.
"I have been fooled enough [in past trials] ... that you really can't predict what you might see. The worst thing in the world you could do is to let something widely out before you have tested safety. That would violate scientific and ethnical principles."
The vaccine would ultimately take the form of one or two shots for people at high risk of contracting Ebola, such as healthcare workers.
"This is not geared toward after someone gets infected," Fauci said. "It is purely, in our minds, a preventative vaccine to be given prior to infection."