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Digital proximity tracing app notifications lead to faster quarantine in non-household contacts: results from the Zurich SARS-CoV-2 Cohort Study

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This study, available as a preprint and thus not yet peer reviewed, examined 393 index COVID-19 cases and 261 close contacts in Switzerland, and found that a digital contact tracing application, compared to manual contact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

Prospective Cohort

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.

This review was posted on: 12 March 2021