Abstract
Non-pharmaceutical interventions are crucial to mitigate the COVID-19 pandemic and contain re-emergence phenomena. Targeted measures such as case isolation and contact tracing can alleviate the societal cost of lock-downs by containing the spread where and when it occurs. To assess the relative and combined impact of manual contact tracing (MCT) and digital (app-based) contact tracing, we feed a compartmental model for COVID-19 with high-resolution datasets describing contacts between individuals in several contexts. We show that the benefit (epidemic size reduction) is generically linear in the fraction of contacts recalled during MCT and quadratic in the app adoption, with no threshold effect. The cost (number of quarantines) versus benefit curve has a characteristic parabolic shape, independent of the type of tracing, with a potentially high benefit and low cost if app adoption and MCT efficiency are high enough. Benefits are higher and the cost lower if the epidemic reproductive number is lower, showing the importance of combining tracing with additional mitigation measures. The observed phenomenology is qualitatively robust across datasets and parameters. We moreover obtain analytically similar results on simplified models.
Highlights
The coronavirus disease (COVID-19) epidemic was declared a pandemic by the World Health Organization on 11 March 2020
The relative impact of contact tracing is larger for lower reproduction numbers, and its cost lower: lower R0 means a slower epidemic reaching fewer individuals even without interventions, so that contact tracing needs to be applied to fewer cases, and fewer people need to be quarantined to mitigate the epidemic
Contact tracing becomes more efficient in a situation where the epidemic is already partially mitigated by other measures that keep R0 as low as possible, showing the importance of combining contact tracing with other measures such as masks or hand hygiene, and that contact tracing might lose efficiency at too large R0
Summary
The coronavirus disease (COVID-19) epidemic was declared a pandemic by the World Health Organization on 11 March 2020. In the first half of 2020, faced with an exponentially growing number of cases and given the absence of effective pharmaceutical treatment and lack of vaccine, governments have first had to rely on broad, nationwide measures to reduce the mobility and number of contacts between individuals, starting with school closures and eventually lockdowns of whole countries [2,3,4,5]. These nonpharmaceutical interventions (NPIs) have succeeded in limiting contagion [6] and they have been gradually lifted in many cases after the first wave. Exit strategies after the end of a lockdown are clearly needed to avoid new resurgences until vaccination has reached sufficient fractions of the population
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