In the race for a Covid-19 vaccine, Merck & Co. looks slow. The drugmaker formally announced its plans just after Memorial Day, shortly before America’s death toll from the coronavirus passed 100,000. It says human trials of one of its shots will begin in mid-August, months behind the likes of AstraZeneca, Moderna, and Pfizer, whose vaccines have already begun large-scale testing. Pfizer has said it could approach regulators for an emergency use authorization as early as October. Merck told Congress last month that 2021 is the absolute earliest it expects to have a vaccine ready, and its top executives have cast doubt on the rosier predictions being made for other shots.