A number of health systems, including in Maryland and D.C., have moved to require coronavirus vaccines for their employees, arguing that they are essential to protect staff and vulnerable patients. The strategy has boosted vaccination rates: More than 2,000 employees at the University of Pennsylvania Health System in Philadelphia have received shots since the system announced in May that it would require all staff members to be vaccinated by September, said Patrick J. Brennan, the system’s chief medical officer.