Nursing shortages have long vexed hospitals. But in the year and a half since its ferocious debut in the United States, the coronavirus pandemic has stretched the nation’s nurses as never before, testing their skills and stamina as desperately ill patients with a poorly understood malady flooded emergency rooms. They remained steadfast amid a calamitous shortage of personal protective equipment; spurred by a sense of duty, they flocked from across the country to the newest hot zones, sometimes working as volunteers. More than 1,200 of them have died from the virus.

Now, as the highly contagious Delta variant pummels the United States, bedside nurses, the workhorse of a well-oiled hospital, are depleted and traumatized, their ranks thinned by early retirements or career shifts that traded the emergency room for less stressful nursing jobs at schools, summer camps and private doctor’s offices…

The staffing shortages have a hospital-wide domino effect. When hospitals lack nurses to treat those who need less intensive care, emergency rooms and I.C.U.s are unable to move out patients, creating a traffic jam that limits their ability to admit new ones. One in five I.C.U.s are at least 95 percent capacity, according to an analysis by The New York Times, a level experts say makes it difficult to maintain standards of care for the very sick.