By Bloomberg When a handful of Dutch health workers fell ill days after the Netherlands’ first Covid-19 case, it prompted mass screening at two hospitals. What scientists found surprised them. Some 1,353 hospital staff in Breda and Tilburg, who recently suffered typical winter coughs and sniffles, were tested for the coronavirus. Of those, 86 -- or 6.4% -- were positive. Barely half had a fever, and the majority reported working while they were mildly ill. The “unexpected high prevalence” indicated hidden community spread, Marion Koopmans , Jan Kluytmans , and colleagues said in a report on Friday. The research is the first to describe clinical effects of the pandemic-causing disease among health-care workers and “confirms the insidious nature” of the coronavirus. It shows “how mild this infection can be,” Koopmans, head of the viroscience department at the Erasmus University Medical Center in Rotterdam, said in a phone interview. “And, therefore, how easily it can fly under the radar.” Staff, who volunteered to be tested and were among more than 9,700 employees at the cities’ major hospitals, didn’t meet the internationally recommended criteria used to define a Covid-19 case at the time of the study, the researchers said. Even if a history of travel to China or northern Italy were excluded from the criteria, 40% of infected health workers identified in the screening still wouldn’t have otherwise been detected, they said. Given the need to swiftly identify and isolate people infected with the virus to stop its spread, the researchers recommend a broader case-definition be applied to identify suspected Covid-19 in health-care workers. Fever should be seen as one of the possible symptoms, and not as a required symptom, the authors said. They also suggested adding severe muscle aches and general tiredness to the case-definition used to identify potentially-infected health workers. Shortages of personal protective equipment is one of the most urgent threats to the outbreak response, World Health Organization Director-General Tedros Adhanom Ghebreyesus told reporters on Friday. “When health workers are at risk, we are all at risk,” Tedros said.