1. Our take

    This study, available as a preprint and thus not yet peer reviewed, examined 393 index COVID-19 cases and 261 close contacts in Switzerland, and found that a digital contact tracing application, compared to manual contact tracing, was associated with a shorter time to quarantine following SARS-CoV-2 exposure among non-household contacts. However, use of the app was not randomized, and users may have been more motivated and adherent with respect to public health recommendations than non-users.

    Study design


    Study population and setting

    Between August 7 and September 30, 2020, index case COVID-19 patients (N=393) and elicited close contacts (N=261) enrolled in the Zurich SARS-CoV-2 Cohort Study (Switzerland) were queried about SARS-CoV-2 exposures and COVID-19 symptoms. They were also asked about their use of SwissCovid, a digital proximity contact tracing application that provides automatic notification of possible SARS-CoV-2 exposures to registered users within geographic proximity of a confirmed COVID-19 case. To evaluate SwissCovid’s effectiveness, the authors assessed the time from SARS-CoV-2 exposure to quarantine among close contacts of index cases, comparing frequent and occasional users of the SwissCovid app to non-app users (who were notified of possible SARS-CoV-2 exposure through manual contact tracing programs). Secondary compliance measures – including proportion of index cases uploading disease notification codes to the SwissCovid app, proportion of close contacts who received a SwissCovid exposure notification, and proportion of notified close contacts reached by SwissApp before manual contact tracing – were also examined.

    Summary of main findings

    Of the 243 (62%) index cases using SwissCovid, 88% received and uploaded a code in the app to trigger an exposure notification. Of the 192 enrolled contacts reporting SwissCovid app use, only one third (38%) received the SARS-CoV-2 exposure notification within 7 days of most recent exposure, of whom only 12% were notified via SwissCovid prior to being contacted by the manual contact tracing program. A majority (67%) of those notified via SwissCovid took no further action, as most had already been contacted by the manual contact tracing program and were quarantined or had tested for SARS-CoV-2. The median number of days from SARS-CoV-2 exposure to quarantine was 2 days (IQR: 1-3 days) among close contacts, and non-household contacts exhibited a longer median time from exposure notification to quarantine (3 days) compared to household contacts (1 day). The probability of initiating quarantine faster was 53% higher among SwissCovid app users relative to non-app users (adjusted Hazard Ratio 1.53, 95% CI: 1.15–2.03). Non-household contacts who were SwissCovid app users had a significantly shorter duration between exposure and quarantine relative to non-users (median 2 days vs. 3 days), but no such relationship was seen for household contacts.

    Study strengths

    The authors assessed both compliance and performance indicators to evaluate the SwissCovid app’s effectiveness on key contact tracing outcomes. The authors also repeated their primary analyses but substituted self-reported exposure history with a proxy measure of estimated SARS-CoV-2 exposure (10 days prior to the last day of quarantine), enabling investigators to estimate potential sources of discord between self-reported exposure history and most likely exposure history estimated from contact tracing data.


    The authors grouped frequent and occasional SwissCovid app users together in analysis, which could have produced misleading estimates of the app’s effectiveness if frequent and occasional app users differed. Additionally, because data on exposure histories and quarantine initiation were self-reported, study findings are susceptible to recall and response biases. Use of the SwissCovid app was not randomized; users of SwissCovid may have been more motivated and concerned about COVID-19 exposure than non-users, which might have biased effect estimates in favor of faster quarantine. There may have been other unmeasured factors (e.g., political affiliation) that confounded the relationship between SwissCovid app use and quarantine initiation. Lastly, because the study was implemented at a time when Switzerland’s SARS-CoV-2 burden was low, results may differ with higher COVID-19 burdens or in populations with different demographic compositions.

    Value added

    This is among the first studies to assess the comparative effectiveness of digital proximity contact tracing applications, relative to manual contact tracing programs, in expediting time to SARS-CoV-2 exposure notification and quarantining among close contacts.

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  2. SciScore for 10.1101/2020.12.21.20248619: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    Institutional Review Board StatementConsent: Informed consent was obtained from all individuals agreeing to participate in the study.
    IRB: In this analysis, we used data from index cases and close contacts enrolled between 07 August 2020 and 30 September 2020, when conditions changed due to a sharp increase in case numbers in Switzerland in early October 2020.21 The study protocol was approved by the ethics committee of the Canton of Zurich (BASEC 2020-01739) and prospectively registered on the International Standard Randomised Controlled Trial Number Registry (ISRCTN14990068).
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.
    Sex as a biological variablenot detected.

    Table 2: Resources

    Software and Algorithms
    Data collection: Data was collected and managed through the Research Electronic Data Capture (REDCap) system.
    suggested: (REDCap, RRID:SCR_003445)

    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: We detected the following sentences addressing limitations in the study:
    Some further limitations of our study should be noted. Selection effects during enrollment may have led to a generally more health literate or compliant study population. However, such self-selection effects would not invalidate the proof-of-principle of our analysis but limit the transportability of our findings to the general population. Furthermore, despite consistent signals in our data, a causality between app notification and faster quarantine could not be unequivocally demonstrated. But the observation of a small subgroup of contacts who received the app notification and entered quarantine before being reached by MCT instills confidence that SwissCovid, in principle, achieves one of its main goals. Further studies are needed to quantify the impact of our findings on pandemic mitigation. To our knowledge, our study is the first to evaluate the real-world effectiveness of a DPT app and leverages data from a prospective population-based cohort study. While a more in-depth assessment of the exact sequence and timing of events related to the notification cascade may shed further light on the impact of SwissCovid on an individual level, our findings confirm the hypothesized benefit of DPT apps alarming non-household contacts earlier than MCT, thereby leading to earlier quarantine.

    Results from TrialIdentifier: We found the following clinical trial numbers in your paper:


    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.

    About SciScore

    SciScore is an automated tool that is designed to assist expert reviewers by finding and presenting formulaic information scattered throughout a paper in a standard, easy to digest format. SciScore checks for the presence and correctness of RRIDs (research resource identifiers), and for rigor criteria such as sex and investigator blinding. For details on the theoretical underpinning of rigor criteria and the tools shown here, including references cited, please follow this link.

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