Merck said Thursday that it has seen “encouraging” results in a clinical trial of an antiviral pill to treat Covid-19 early in the disease’s course. At the same time, the company said it will stop efforts to develop both that drug and a second medicine for patients who have already been hospitalized with the disease. The drug giant licensed the anti-Covid pill, molnupiravir, from Ridgeback Biotherapeutics last July.