Nearly one in four beds lies empty at TaraVista Behavioral Health Center in central Massachusetts — not for lack of patients, but for lack of staff.

Even before the pandemic, nurses and lower-paid aides were in perennially short supply, but the 116-bed facility could still run full, said Chief Executive Officer Michael Krupa. At similar hospitals around Massachusetts, hundreds of beds can’t be filled, and "the reason is exclusively staff,” he said.

What some are calling the worst U.S. health care labor crisis in memory is sharpening concerns about attrition from resistance to vaccine mandates — even in the medical mecca of Massachusetts, where COVID-19 cases remain well within hospital capacity. About 16% of American hospitals had critical staffing shortages as of Oct. 1, according to the U.S. Department of Health and Human Services.