Regeneron Pharmaceuticals said Monday that a single shot of its monoclonal antibody cocktail reduced the risk that volunteers exposed to Covid-19 would develop the disease by 81%. The study enrolled 1,500 healthy volunteers, each of whom shared a home with someone who tested positive for SARS-CoV-2, and randomized them to receive a single dose of its antibody treatment, given subcutaneously, or placebo. After 29 days, 11 patients in the treatment group developed Covid-19 compared to 59 on placebo. And for the subjects who got Covid-19 despite treatment, their symptoms resolved after one week, compared to three weeks for those on placebo.