Most COVID-19 infections among hospital healthcare workers occurred in low-contagion areas among younger nurses, a new study from China has found. Researchers suggest that these providers were unknowingly at risk and less protected from asymptomatic carriers.

Among more than 9,600 health care workers at Tongji Hospital, Wuhan, China, 110 developed COVID-19 early in the disease outbreak. Younger workers in the hospital’s low-contagion wards — especially nurses under age 45 — had a much higher rate of infection than older workers who were assigned to care for COVID-19 patients in separate wards.

Investigators attributed this counterintuitive outcome to a lack of disease knowledge early in the outbreak, and less protective gear provided to staff in the non-COVID hospital wards at the time. But the biggest likely risk to nurses and other providers were asymptomatic patients and staff, they concluded. Many patients who were infected had no or very subtle symptoms, and some exhibited atypical symptoms.

“The existence of such patients could greatly endanger the health of staff even though clinical areas caring for patients with and without COVID-19 were separated from each other,” wrote first author Xiaoquan Lai, M.D., and colleagues.

Asymptomatic staff may also have contributed to transmission, they added. “Most healthcare workers with infection had mild symptoms; however, special attention needs to be paid to protect healthcare workers from cross-infection from other healthcare workers,” the authors wrote.

The findings are a reminder to remain vigilant, especially about worker-to-worker transmission, they concluded. “Rapid identification of staff with potential infection and routine screening among asymptomatic staff could help protect healthcare workers.”

The study was published last week in JAMA Network Open.