The drug company, along with a German partner, is running tests in healthy volunteers. It’s one of several companies on an accelerated timetable to try to find a safe, effective vaccine. Pfizer and the German pharmaceutical company BioNTech announced that their potential coronavirus vaccine began human trials in the United States on Monday. If the tests are successful, the vaccine could be ready for emergency use here as early as September. The two firms are jointly developing a vaccine candidate based on genetic material known as messenger RNA, which carries the instructions for cells to make proteins. By injecting a specially designed messenger RNA into the body, the vaccine could potentially tell cells how to make the spike protein of the coronavirus without actually making a person sick.