TRENTON, N.J. (AP) — The U.S. and Liberian governments have just begun the first formal patient testing of an experimental Ebola virus treatment that’s previously been used only on an emergency basis.

The drug, ZMapp, contains three genetically engineered proteins designed to hone on a target on the surface of the deadly virus to stop the disease’s progression. ZMapp, developed by San Diego-based Mapp Pharmaceuticals Inc., is “grown” in tobacco plants engineered to make large quantities of the virus-blocking proteins.

Adults, as well as children of any age, will be enrolled in the study if they are admitted to Ebola treatment units in Liberia or are health- care workers who have returned to the U.S. for treatment after being infected with Ebola while serving in West Africa.

In addition, adults and children who may have acquired Ebola in the United States from contact with an infected person will be enrolled. That has happened to only a handful of Americans so far, most notably two nurses who treated patient Thomas Eric Duncan, who had traveled from Africa and died at a Dallas hospital.

The National Institute of Allergy and Infectious Diseases said Friday that half the participants will receive three ZMapp injections once a day for three days, with the dose based on the person’s weight, along with supportive treatment. The other half, the control group, will only receive supportive treatment, including intravenous fluids, medication for any other infections they get and therapy to maintain blood pressure and sufficient oxygen intake.

Each participant will be monitored for up to a month after hospital discharge, including any follow-up outpatient treatment, and researchers then will compare how well the two groups fare. Such studies — with a control group and patients randomly assigned to one group or another — are considered the gold standard for evaluating a treatment’s value.

The goal is to enroll 100 patients in each group, though that may be difficult because there have been so few U.S. cases and the epidemic is waning in Africa, particularly in Liberia. Each patient must agree to participate after the study is fully explained to them.

The study is expected to wrap up in December 2016.

The goal is to then conduct similar studies of five additional experimental Ebola treatments. Meanwhile, at least three potential vaccines against Ebola also are in testing.

ZMapp has previously been given under emergency authorization to a total of nine infected patients.