A smaller dose would provide immunity for at least 12 months, they said.
Emergency stocks have been depleted by a mass immunisation programme in Angola, where the disease has killed more than 300 people since December.
A surge of cases in the Democratic Republic of Congo has raised fears of a crisis situation.
The current outbreak of the disease has seen 18 million vaccine doses distributed in Angola, DR Congo and Uganda.
The global stockpile of the vaccine has already been depleted twice since February.