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Outcomes of Contact Tracing in San Francisco, California—Test and Trace During Shelter-in-Place

Our take —

In San Francisco, California, only half of reachable COVID-19 index cases between April 13 and June 5, 2020, reported >1 close contacts, 84% of whom were notified of their exposure. Of notified contacts, 45% were tested for SARS-CoV-2 infection; 26% of those tested were positive. These results, and the average 6-day delay between case symptom onset and contact notification, provide reasons for concern about the effectiveness of contact tracing for transmission control in San Francisco, at least during the study period. Because contact tracing outcomes were estimated during a municipal shelter-in-place ordinance, these outcomes may not reflect performance of a contact tracing program after restrictions were lifted.

Study design


Study population and setting

The authors aggregated all cases of confirmed COVID-19 in San Francisco, California, during the shelter-in-place order from April 13 to June 5, 2020. Close contacts (both household and non-household) were identified from index cases and from May 5 onwards and were referred for COVID-19 testing irrespective of symptoms. The authors quantified outcomes along the contact tracing continuum using descriptive statistics.

Study Strengths

The authors enumerated specific outcomes in the contact tracing pipeline (e.g., number of contacts identified, number of close contacts notified/tested, median time to contact notification) to assess the effectiveness of contact tracing strategies in San Francisco. Additionally, the authors disaggregated contact tracing outcomes by contact status (household versus non-household contacts) and compared summary statistics across racial/ethnic identities.


The authors did not describe in detail procedures for reaching close contacts, including communication modalities used or number of contact attempts before close contact was deemed lost-to-follow-up. Because close contacts were identified during a shelter-in-place ordinance, index cases may have been reluctant to report non-household contacts to the health department; the number of non-household contacts was, therefore, likely underestimated. Data collection during a shelter-in-place ordinance may also not be reflective of health system performance or capacity once these restrictions are lifted. Lastly, because the authors did not enumerate any health system performance metrics (e.g., number of contact tracing staff), presented contact investigation outcomes cannot be fully attributed to health system performance or public health workforce capacity.

Value added

This is among the first studies in the United States to describe outcomes related to COVID-19 contact tracing efforts during the early months of the U.S. epidemic in a large metropolitan area.

This review was posted on: 20 November 2020