Almost one in five hospital patients was injured by their care, according to a study of 10 U.S. hospitals that found that industry and government efforts to improve safety produced insignificant changes.

The six-year study of 2,341 hospital admissions in North Carolina found that 18 percent of patients suffered at least one safety-related incident, ranging from minor injuries with little harm to life-threatening mistakes and fourteen deaths.

The rate of injuries did not decrease significantly from 2002 to 2007, researchers reported in the study published Wednesday in the New England Journal of Medicine.

Efforts to improve patient safety intensified in the U.S. after a 1999 report by the Institute of Medicine found that medical errors caused as many as 98,000 deaths and more than 1 million injuries each year. Christopher Landrigan, a researcher at Brigham and Women’s Hospital in Boston who led the study, says “The rate is high, but it’s not higher than we expected.”