This report from the CDC describes how a sentinel surveillance system for COVID-19 cases helped confirm community transmission in Santa Clara County, CA.

On February 27, 2020, Santa Clara County, California, identified its first case of coronavirus disease 2019 (COVID-19) associated with probable community transmission. Testing guidance originally recommended focusing on persons with clinical findings of lower respiratory illness and travel to an affected area or an epidemiologic link to a laboratory-confirmed COVID-19 case, or on persons hospitalized for severe respiratory disease and no alternative diagnosis.

To rapidly understand the extent of COVID-19 in the community, SCCPHD, the California Department of Public Health (CDPH), and CDC began sentinel surveillance in Santa Clara County in four urgent care centers. For this investigation, county residents evaluated for respiratory symptoms (e.g., fever, cough, or shortness of breath) who had no known risk for COVID-19 were identified at participating urgent care centers.

During March 5–14, 226 patients with respiratory symptoms were evaluated at one of these sites; 23% had positive test results for influenza. Among patients who had negative test results for influenza, 79 specimens were tested for SARS-CoV-2, and 11% had evidence of infection.

COVID-19 cases identified through this sentinel surveillance system helped confirm community transmission in the county. As a result of these data and an increasing number of cases with no known source of transmission, the county initiated a series of community mitigation strategies.

Local health departments can use sentinel surveillance to understand the level of community transmission of COVID-19 and to better guide the selection and implementation of community mitigation measures.