Changes in social contact time following digital SARS-CoV-2 post-exposure notification: A retrospective case-control study at a US university

Abstract

Abstract In the first year of the COVID-19 pandemic, digital contact tracing smartphone applications were developed for the purpose of reducing transmission by encouraging individuals to voluntarily quarantine after exposure. Privacy protocols used in many of these apps prevented longitudinal linkage of users’ contact network data, making it impossible to observe whether users reduced social contact following digital exposure notification. We used anonymous data from one particular app deployment, which used alternative privacy measures and retained longitudinal interaction data, to evaluate changes in the users’ time in proximity to other users after digital exposure notification. We analyzed a dataset of the anonymous app users in a US university community January − March 2021, before COVID-19 vaccines were widely available. Users provided consent for their anonymous data to be used for research. We conducted pre-post analysis of contact time with other users before and after using the app to self-report being a positive COVID-19 case, and before and after exposure notification by the app. We further conducted a retrospective case-control study by matching users who did and who did not receive exposure notification based on having similar contact time with other users prior to notification. In pre-post analysis, we found that users who reported being a positive case to the app reduced their total contact time with others by 29% (p = 0.002) in the 7-day period immediately after time of the report vs. the 7-day period before the time of the report. However, we observed no statistically significant change in contact time before vs. after exposure notification. We also observed no statistically significant difference in contact time compared to matched controls who received no notification. Our findings suggest that digital contact tracing app users affiliated with this US university did not reduce contact with others after receiving exposure notification. Our study provides one of the first rigorous evaluations of the effectiveness of digital exposure notifications during a time when communities were reliant on non-pharmaceutical interventions to reduce disease spread.