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The drug company Chimerix has abruptly stopped a clinical trial underway in Monrovia, Liberia. Chimerix’s decision isn’t due to any known problem with its product, the antiviral drug brincidofovir. Rather, it’s because those in charge of the trial can’t find enough Ebola-infected people to participate.

Last week, the World Health Organization reported that the number of new Ebola cases in Liberia, Guinea, and Sierra Leone had plummeted to below 100 in one week—the lowest count since June of last year. And in the week ending Sunday, January 25, only four new cases had arisen in Liberia.

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An Ebola treatment tent stationed in Guinea

The current Ebola outbreak’s apparent demise is surely a good thing, but it does limit the extent to which scientists can evaluate existing treatments to prepare for the next one. Brincidofovir, like other drugs in the works, cannot be put to use without ensuring its effectiveness and safety by testing it on a certain number of patients. This particular trial would have ideally administered the drug to about 140 people, but only ten have been treated since January 2.

Brincidofovir isn’t the only treatment that’s being stopped in its tracks. Doctors Without Borders has been collecting blood plasma from survivors to see if infusions can aid those suffering from the disease. Two vaccines, as well as the drugs ZMapp and Avigan, have clinical trials that are either impending or in progress. All of these trials could risk termination if Ebola cases become more and more rare.

Here’s Andrew Pollack, reporting for The New York Times:

Dr. [Armand] 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. Related A Short History of Quarantine One Man's Battle With Anorexia Two Sexes Are Not Enough 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.

With a drop in Ebola patients, hope is on the horizon that this outbreak is nearly over. But public health officials need to stay alert. Outbreaks can have sudden, unexpected resurgences, and until scientists have a clear understanding of how the Ebola virus mutates and what are the best ways to protect against it, they won’t be resting easy.

Learn more about the Ebola outbreak and what experts are doing to stop it with "Surviving Ebola."