When Dr. Kent Brantly, the first U.S. Ebola patient, contracted the deadly virus in 2014, physicians raced to give him a highly experimental drug called ZMapp. No one knew whether it would work — it was just a cocktail of mouse antibodies. Thankfully, Brantly, along with many others who took the drug, survived the ordeal. Now, after its first round of clinical trials, ZMapp is being considered the most promising potential treatment to prevent Ebola-related deaths.

Scientists aren’t racing to get it into hospitals just yet, though. ZMapp saved Brantly’s life, but it failed to help many others, prompting doctors to question its efficacy — Brantly, for one, partially chalked his survival up to his faith. A team of researchers set out to test the drug in a rigorous clinical trial and determine once and for all just how well ZMapp can combat Ebola.

The researchers, reporting from the Conference on Retroviruses and Opportunistic Infections in Boston, announced Tuesday the results of their trial: There was a 22 percent mortality rate among patients 72 Ebola patients originating from Sierra Leone, Liberia, Guinea, and the United States, — that is, eight of the 36 patients that received the drug died. In the group of patients that didn’t receive the drug, the mortality rate was 37 percent.

The results were enough for the researchers to say that the drug was promising, but they couldn’t quite say they were conclusive — because statistically speaking, it’s possible the difference in mortality rate is just a product of chance. The only way the researchers could have avoided this statistical hurdle would have been to have more patients in the study, but unfortunately — and also fortunately — there weren’t enough Ebola patients to recruit for the trial before the epidemic had finally subsided.

Although the study was cut short, its results give researchers a reason to press forward with the drug’s development. Mapp Biopharmaceutical, the San Diego company that creates ZMapp, has stated it would continue its push to get the drug approved by the FDA.

Meanwhile, researchers are trying different strategies to attack the virus. One vaccine, developed using antibodies from a human Ebola survivor, also recently showed promising results.

The West Africa Ebola outbreak, which claimed 11,000 lives during its spread, is now considered to be officially over, but researchers are expecting inevitable flare-ups in its wake, making the need for a vaccine more urgent than ever.