Kevzara, which is made by Regeneron and Sanofi, does not attack the novel coronavirus but instead inhibits an abnormal immune response called a "cytokine storm" that causes the lungs of the sickest patients to become inflamed, leaving them fighting for their lives on ventilators.

An early small study in China had appeared promising, but the drug showed no benefit over a placebo in a larger US study of 276 patients with "severe" disease—that is to say, those requiring oxygen but not ventilators.

There was, however, a ray of hope for those who were "critical," defined as needing mechanical ventilation or high-flow oxygenation. In this group, 44 were on a placebo, 94 were given a low dose and 88 were given a high dose.

Fifty-five percent of patients on the placebo died by the end of the study period, compared with 46% on the lower dose and 32% on the high dose.

The trial will continue among this critical group.

"We await results of the ongoing phase 3 trial to learn more about COVID-19, and better understand whether some patients may benefit from Kevzara treatment," said George Yancopoulos, Regeneron's president and co-founder.

The company is separately developing an antibody cocktail that will directly target the virus and that it hopes to advance into human trials by June.

These antibodies are being acquired by infecting mice that have had their immune systems genetically modified to become human-like.

Regeneron has previously used this system to develop a treatment that proved effective against Ebola.