A woman with COVID-19 at a Solano County hospital - the nation's first case from an unknown source - exposed 121 health workers to the coronavirus, yet only three got the disease, a new study of the February case reveals. All three had been in close contact for about two hours with the patient, and two had no protective gear, according to the report published Tuesday by the Centers for Disease Control and Prevention. Those workers also examined the patient and performed treatments that involved close contact, such as placing her on a breathing machine. The researchers tested just 43 of the 121 workers for the coronavirus because only they had developed a cough or other symptoms similar to those of COVID-19 patients within two weeks after exposure. Across the country, more than 9,200 health workers out of an estimated 18 million have the disease, the CDC reported Tuesday in a separate paper, which notes that the number probably understates the true number of coronavirus infections. Dr. Robert Siegel, an infectious disease expert at Stanford, called the relatively low infection rate from the Solano County patient a hopeful sign. But he added that public health officials should remain vigilant in protecting workers. "The results are promising for health care workers. It means that the risk may be less than we thought," Siegel said.