There was no discernible effect of the treatment on those who had generated natural antibody responses.
"People have been very, very sceptical, that any treatment against this particular virus would work by the time people get in hospital," Martin Landray, the joint chief investigator on the trial, told reporters.
"If you haven't raised antibodies of your own, you really would benefit from getting some," he said.
The treatment also shortened the hospital stay of those who were seronegative and reduced their chances of needing a mechanical ventilator, Landray said.