Nested case-control study of health workers exposed to confirmed COVID-19 patients. Similar objectives to the cohort study but case-control studies may be cheaper and provide robust evidence to characterize and assess the risk factors for SARS-CoV-2 infection in health workers exposed to COVID-19 patients. Health workers with confirmed COVID-19 will be recruited as cases and other health workers in the same health care setting without infection will be recruited as controls (incidence density sampling). Secondary objectives are similar to the cohort study.