The vaccine trials began taking shape after the Liberian government, at the height of the epidemic last year, asked the United States to conduct clinical research in Liberia on potential vaccines and drugs. Two vaccines, one manufactured by Merck and another by GlaxoSmithKline, were chosen after initial studies showed they were generally safe and produced an immune response against Ebola in human volunteers in the United States and other countries.

The West African trials are the first time vaccines are being tested in the context of an outbreak, though the waning caseload may make it more difficult to answer the ultimate question — whether they really protect people from contracting Ebola.

In the Liberian trial, expected to last about a year, participants will be given one of the two vaccines, or a placebo, at 10 locations in and around Monrovia.

On Tuesday, researchers finished vaccinating an initial batch of 600 volunteers at Redemption Hospital, which was used as an Ebola holding center a few months ago. Officials said they had made sure that participants fully understood the consent forms — a critical issue in places with high illiteracy and low education levels, as in Liberia. Explanations of risks and potentially unfamiliar concepts, like placebos and randomized trials, are given in Liberian English, or, if need be, in one of the 16 local languages.

For 10 visits over the course of almost a year, the first 600 participants will each be handed $300: $40 on the first visit, less on subsequent visits, and $150 at the end. The money is supposed to compensate participants for transportation costs and lost wages.

“The ethics of it is compensation for inconvenience,” said Dr. Lane, the lead American researcher. “You don’t pay people to be part of medical research.”