The World Health Organization reported last week that the number of new cases in the three most affected countries — Liberia, Guinea and Sierra Leone — had fallen below 100 a week for the first time since June. In Liberia, there were only four new cases in the seven days ending Jan. 25, the organization said.

Researchers said that, of course, the ebbing of the outbreak was a good problem to have.

“It’s more important to end the outbreak than to get the trial done,” said Dr. Armand Sprecher, a public health specialist at Doctors Without Borders, whose Ebola treatment center in Monrovia was the site of the brincidofovir trial.

Doctors Without Borders treatment centers are also being used to study the antiviral drug Avigan and also blood plasma collected from Ebola survivors. Avigan, also known as favipiravir, is a flu drug developed by Fujifilm of Japan.

Dr. Sprecher said the study of Avigan, which is taking place in Guinea, started before the one for brincidofovir and had already treated a “decent” number of patients. Organizers of that trial, however, are now looking for another clinic in order to find more patients, he said. The plasma trial has not really started yet, he said.

Organizers of a different trial of survivors’ plasma that has started in Liberia are now looking for collaborators to expand the testing into Sierra Leone, said Dr. William A. Fischer II, an assistant professor of medicine at the University of North Carolina and an investigator in that study. He said that besides testing the plasma as a treatment, the effort was aimed at improving the capacity of the health systems in West Africa to conduct safe blood donations and transfusions.