Early evidence of effectiveness of digital contact tracing for SARS-CoV-2 in Switzerland
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Abstract
In the wake of the pandemic of coronavirus disease 2019 (COVID-19), contact tracing has become a key element of strategies to control the spread of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Given the rapid and intense spread of SARS-CoV-2, digital contact tracing has emerged as a potential complementary tool to support containment and mitigation efforts. Early modelling studies highlighted the potential of digital contact tracing to break transmission chains, and Google and Apple subsequently developed the Exposure Notification (EN) framework, making it available to the vast majority of smartphones. A growing number of governments have launched or announced EN-based contact tracing apps, but their effectiveness remains unknown. Here, we report early findings of the digital contact tracing app deployment in Switzerland. We demonstrate proof-of-principle that digital contact tracing reaches exposed contacts, who then test positive for SARS-CoV-2. This indicates that digital contact tracing is an effective complementary tool for controlling the spread of SARS-CoV-2. Continued technical improvement and international compatibility can further increase the efficacy, particularly also across country borders.
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SciScore for 10.1101/2020.09.07.20189274: (What is this?)
Please note, not all rigor criteria are appropriate for all manuscripts.
Table 1: Rigor
Institutional Review Board Statement not detected. Randomization not detected. Blinding not detected. Power Analysis not detected. Sex as a biological variable not detected. Table 2: Resources
No key resources detected.
Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).
Results from LimitationRecognizer: An explicit section about the limitations of the techniques employed in this study was not found. We encourage authors to address study limitations.Results from TrialIdentifier: No clinical trial numbers were referenced.
Results from Barzooka: We did not find any issues relating to the usage of bar …
SciScore for 10.1101/2020.09.07.20189274: (What is this?)
Please note, not all rigor criteria are appropriate for all manuscripts.
Table 1: Rigor
Institutional Review Board Statement not detected. Randomization not detected. Blinding not detected. Power Analysis not detected. Sex as a biological variable not detected. Table 2: Resources
No key resources detected.
Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).
Results from LimitationRecognizer: An explicit section about the limitations of the techniques employed in this study was not found. We encourage authors to address study limitations.Results from TrialIdentifier: No clinical trial numbers were referenced.
Results from Barzooka: We did not find any issues relating to the usage of bar graphs.
Results from JetFighter: We did not find any issues relating to colormaps.
Results from rtransparent:- Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
- Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
- No protocol registration statement was detected.
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Our take
This proof-of-concept study, available as a preprint and thus not yet peer reviewed, evaluated uptake and effectiveness of SwissCOVID, a decentralized digital contact tracing application (i.e., app) that maintains individual user privacy, using multiple independent data sources from Switzerland. Results suggest that initial uptake of the app was high (~20% of the total Swiss population) and that individuals sought testing as a result of exposure notifications through the app. However, due to a large amount of missing data and because of the decentralized nature of the app itself, quantifying and ultimately interpreting the app’s effectiveness relative to traditional contact tracing methods was difficult. Overall, the results suggest that decentralized digital contact tracing apps may help supplement routine contact …
Our take
This proof-of-concept study, available as a preprint and thus not yet peer reviewed, evaluated uptake and effectiveness of SwissCOVID, a decentralized digital contact tracing application (i.e., app) that maintains individual user privacy, using multiple independent data sources from Switzerland. Results suggest that initial uptake of the app was high (~20% of the total Swiss population) and that individuals sought testing as a result of exposure notifications through the app. However, due to a large amount of missing data and because of the decentralized nature of the app itself, quantifying and ultimately interpreting the app’s effectiveness relative to traditional contact tracing methods was difficult. Overall, the results suggest that decentralized digital contact tracing apps may help supplement routine contact tracing activities, but more data is needed to determine their effectiveness.
Study design
cross-sectional;prospective-cohort;ecological
Study population and setting
SwissCOVID, a digital contact tracing application (i.e., app) launched by the Switzerland public health authority, was deployed to the general population on June 25, 2020. Authors report early findings on the effectiveness of this Exposure Notification (EN) - based contact tracing system from July 23 to September 10, 2020. This EN framework uses Bluetooth Low Energy (BLE) to determine close proximity of two devices (defined as 1.5 meters) and a single-use validation code (CovidCode) linked to the user’s contagious period provided by the Swiss health authority to notify other users who had spent at least 15 minutes within 1.5 meters of a confirmed COVID-19 case. The app is decentralized, meaning that user data is stored locally on users’ phones and not reported to a central authority, thereby preserving individual privacy. Given the decentralized nature of the app, multiple sources of data were collected to evaluate its effectiveness. App usage was monitored through the Federal Statistical Office and routine public health surveillance data. The Zurich SARS-COV-2 cohort, consisting of 235 index cases and 185 exposed contacts, recruited from August 7 to September 11, 2020, was also used to assess uptake and effectiveness of the app.
Summary of main findings
SwissCOVID was downloaded 2.36 million times by September 10, 2020, with an estimated 1.62 million active apps per day, representing 19% of the Swiss population. Out of the 12,456 reported cases during the study period, 2,447 (20%) were provided CovidCodes for the app, and 1,645 (67%) ultimately entered their diagnosis in the app. 1,695 phone calls were received by the SwissCOVID hotline as a result of an app notification (evidence that exposed contacts reacted to the reception of notification). The authors also used routine public health surveillance data and multiple imputation by chained equations (a statistical approach to account for missing data) to estimate that 65 confirmed COVID-19 cases seeked testing as a result of being notified through the SwissCOVID app. Using these same data, the effectiveness of the app was estimated to be similar to that of traditional contact tracing methods using traditional contact tracing data from South Korea and Taiwan. Lastly, 63% (148/235) of confirmed cases in the Zurich SARS-Cov-2 cohort used the app. Among these cases, 127 (86%) inputted their CovidCode diagnosis into the app. 132 of the cohort’s 185 exposed contacts (71%) used the app, of which only 46 (35%) received notification of exposure through the app.
Study strengths
In this large national study, the authors use multiple independent data sources to evaluate the effectiveness of a digital contact tracing application.
Limitations
There are several notable limitations to this study. First, the total number of users reached through notifications once a confirmed case logs their CovidCode into the app was unknown, limiting our understanding of the reach of digital contact tracing. Second, calls to the SwissCOVID hotlines might not necessarily reflect the adherence of exposed contacts to quarantine recommendations post-notification. Third, it is unclear why out of the 12,456 confirmed cases, only 2,447 were issued a CovidCode. Fourth, information on the time between testing and receiving and entering a CovidCode was not provided, limiting our understanding of digital contact tracing effectiveness. Fifth, there are limitations to the authors’ inference about app usage using routine testing data: only 63% of confirmed cases were asked why they were getting tested; participants could also have reported the appearance of symptoms as the primary reason for getting tested instead of the app itself; and notified contacts may not necessarily have gotten tested, particularly if they were asymptomatic. Finally, the effectiveness of the app in the study population may not necessarily reflect the larger Swiss population. Information about the types of contacts notified through the app (e.g., household, workplace) was not reported, which would have improved understanding of the app’s reach.
Value added
This is among the first studies to report on the effectiveness of a digital contact tracing application and its potential for mitigating the COVID-19 pandemic.
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