Estimation of effects of contact tracing and mask adoption on COVID-19 transmission in San Francisco: a modeling study

Abstract

The current COVID-19 pandemic has spurred concern about what interventions may be effective at reducing transmission. The city and county of San Francisco imposed a shelter-in-place order in March 2020, followed by use of a contact tracing program and a policy requiring use of cloth face masks. We used statistical estimation and simulation to estimate the effectiveness of these interventions in San Francisco. We estimated that self-isolation and other practices beginning at the time of San Francisco's shelter-in-place order reduced the effective reproduction number of COVID-19 by 35.4% (95% CI, -20.1%--81.4%). We estimated the effect of contact tracing on the effective reproduction number to be a reduction of approximately 44% times the fraction of cases that are detected, which may be modest if the detection rate is low. We estimated the impact of cloth mask adoption on reproduction number to be approximately 8.6%, and note that the benefit of mask adoption may be substantially greater for essential workers and other vulnerable populations, residents return to circulating outside the home more often. We estimated the effect of those interventions on incidence by simulating counterfactual scenarios in which contact tracing was not adopted, cloth masks were not adopted, and neither contact tracing nor cloth masks was adopted, and found increases in case counts that were modest, but relatively larger than the effects on reproduction numbers. These estimates and model results suggest that testing coverage and timing of testing and contact tracing may be important, and that modest effects on reproduction numbers can nonetheless cause substantial effects on case counts over time.

Commentary: Physiological and Psychological Impact of Face Mask Usage during the COVID-19 Pandemic

“…In the United States, as of July 2020, the CDC recommends “that people wear cloth face coverings in public settings when around people outside of their household, especially when other social distancing measures are difficult to maintain” [ 4 ]. Contrary to what some may believe, these cloth masks/face coverings are not designed to protect the mask wearer, but rather, they are intended to protect people within close proximity of the wearer [ 8 ].…”

“…A recent study of COVID-19 in San Francisco suggested that the most effective public interventions in reducing virial spread were sheltering in place and contact tracing efforts. However, once shelter in place procedures were lifted, adopting the use of cloth masks by the community was deemed an important and effective means of reducing virus spread [ 8 ]. Chu and colleagues’ recent meta-analysis of 172 observational studies examined the impact of face masks and physical distancing on the transmission of SARS-CoV-2, and reported that face masks (in addition to physical distancing) decreased the transmission of the virus [ 16 ].…”

Effectiveness of face masks worn in community settings at reducing the transmission of SARS-CoV-2: A rapid review

“…As noted by the authors, these findings describe the intention-to-treat effect of face mask mandates at a community or state level, and do not indicate the effect of wearing a face mask in public at an individual level 37 . A number of studies have reported similar positive findings, suggesting that policies mandating face masks are associated with a subsequent decrease in COVID-19 cases 38,39 . However, these policies are rarely introduced in isolation, and it can be difficult to disentangle the effects of multiple measures targeting reduced transmission.…”

The role of masks in reducing the risk of new waves of COVID-19 in low transmission settings: a modeling study

“…A study from Israel showed that masks are particularly effective when the effective R is close to 1, and can determine whether a low-level epidemic tips into an outbreak or not [22]. In higher transmission settings where the effective R is greater than 1, two studies found that masks are most effective when used in conjunction with a collection of other NPIs [9], [23]. To our knowledge, no studies to date have examined the extent to which face masks can prevent a resurgent outbreak in low-transmission settings.…”