For the first time, preliminary clinical-trial results suggest that two experimental Ebola drugs can lower the death toll of the deadly virus, health officials announced Monday.

Two other experimental drugs used in the trial were less effective and will be abandoned.

The data comes from the PALM trial, which is short for the Swahili phrase Pamoja Tulinde Maisha, meaning Together Save Lives. The trial began in late 2018 amid the Ebola outbreak in the Democratic Republic of Congo, which is still ongoing and is now the second-largest outbreak on record. Ebola responders in the DRC aimed to enroll 725 patients, but they only used data from 499 for the preliminary analysis of the results.

The trial tested three experimental drugs against ZMapp. The therapy is a pre-existing cocktail of antibodies that had shown promise at treating Ebola in an earlier outbreak. But researchers hadn't been able to get enough data to tell if it actually prevented deaths.

Still, with no other Ebola-specific treatments in the game, ZMapp had until this point been considered the standard of care, despite the inconclusive data.

ZMapp's competition in the PALM trial included the monoclonal antibody mAb114. Generally, antibodies are Y-shaped proteins made by cells in the blood to bind specific pathogens—such as Ebola—in an effort to destroy the invader. mAb114 was developed by researchers at the National Institute of Allergy and Infectious Diseases from an antibody isolated from a survivor of an Ebola outbreak in 1995.

PALM also included REGN-EB3, a cocktail of three monoclonal antibodies developed by Regeneron Pharmaceuticals from antibodies harvested from Ebola-infected mice with "humanized" immune systems.

The last of the four drugs tested was the antiviral drug remdesivir, made by Gilead.

From the trial's early data, 49% of patients given ZMapp died, as did 53% of those receiving remdesivir. By contrast, only 29% of patients treated with REGN-EB3 died, and only 34% of patients treated with mAb114 died.

The fatality rate of the current outbreak is estimated to be around 70%.

Moreover, when researchers looked at how patients fared when they sought treatment early in the disease progression, the drugs looked even better. Only 6% of those treated early with REGN-EB3 died, and 11% of those treated early with mAb114 succumbed to the disease. Mortality rates for early treatments with ZMapp and remdesivir were 24% and 33%, respectively.

Moving forward, researchers will only carry on with REGN-EB3 and mAb114 in further trials.

So far in the current outbreak, Ebola has infected around 2,800 people and killed nearly 1,900. Earlier work in the outbreak suggested that an experimental vaccine against the virus is 97.5% effective.