Lessons from Mass-Testing for COVID-19 in Long Term Care Facilities for the Elderly in San Francisco.

COVID-19 can cause significant mortality in the elderly in Long Term Care Facilities (LTCF). We describe four LTCF outbreaks where mass testing identified a high proportion of asymptomatic infections (4-41% in health care workers and 20-75% in residents), indicating that symptom-based screening alone is insufficient for monitoring for COVID-19 transmission.