A monoclonal antibody treatment for Covid-19 developed by Regeneron saved lives among hospitalized patients who had not mounted their own immune response, a finding that could dramatically change the way that doctors will use the therapy, researchers in the UK announced Wednesday. The new data mark the first time that a medicine that works by fighting the SARS-CoV-2 virus has been shown to reduce mortality. Other treatments, such as the steroid dexamethasone, have been shown to save lives by tamping down the immune system’s overactive response to the virus.