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Up to 6% of the staff caring for patients with COVID-19 at 13 hospitals across the country tested positive for the virus -- but nearly 30% of them were asymptomatic -- according to a report released Monday by the U.S. Centers for Disease Control and Prevention.
Researchers say the doctors, nurses, and physicians assistants who remained asymptomatic may have spread the infection unknowingly to patients and coworkers.
Based on these findings, enhanced screening in hospitals -- including frequent testing of front-line health workers and universal use of face coverings and personal protective equipment, or PPE -- can help reduce the spread of the disease, the report's authors said.
"[Our] study resulted in the identification of two factors potentially associated with [COVID-19] infection among healthcare personnel: PPE shortages and interacting with patients without wearing a face covering," the CDC researchers wrote.
"Universal masking has been associated with a significantly lower rate of infection among healthcare personnel," they said.
The report's findings are based on an analysis of 3,248 healthcare workers at the 13 hospitals, all of whom underwent blood tests for COVID-19 antibodies -- immune cells that help the body fight off viruses -- between April 3 and June 19.
Nearly 200 of the workers were found to have antibodies for the virus, meaning they had been infected sometime after it first came to the United States in early February.
Of the hospitals, Montefiore Medical Center in New York City had the highest percentage of staff members with COVID-19 antibodies at 31%, while Ohio State University's Wexner Medical Center had the lowest, less than 1%, researchers said.
New York City was the epicenter of the COVID-19 outbreak in the United States in March, April, and May.
Among those who tested positive, 29% reported never having symptoms of COVID-19 and 69% said they were not diagnosed with the disease, the researchers said.
They found that the prevalence of virus antibodies was higher, at 9%, among staff who reported not always wearing a face-covering -- either a surgical mask, N95 respirator, or powered air-purifying respirator.
"Healthcare personnel who care for patients with COVID-19 are at risk for exposure and infection during patient care-related activities and once infected can spread the virus," the CDC researchers wrote.
"Understanding the frequency of infection among front-line [workers] ... is important for planning effective strategies for minimizing spread in healthcare settings."