An investigational antiviral pill reduced the chances that patients newly diagnosed with Covid-19 would be hospitalized by about 50%, a finding that could give doctors a desperately needed new way to treat the sick, the drug maker Merck announced Friday. A five-day course of molnupiravir, developed by Merck and Ridgeback Biotherapeutics, reduced both hospitalization and death compared to a placebo. In the placebo group, 53 patients, or 14.1%, were hospitalized or died. For those who received the drug, 28, or 7.3%, were hospitalized or died.