BACKGROUND: Contact tracing is one of the most effective non-pharmaceutical interventions in the COVID-19 pandemic. This study uses a multi-agent model to investigate the impact of four types of contact tracing strategies to prevent the spread of COVID-19. METHODS: In order to analyse individual contact tracing in a reasonably realistic setup, we construct an agent-based model of a small municipality with about 60.000 inhabitants (nodes) and about 2.8 million social contacts (edges) in 30 different layers. Those layers reflect demographic, geographic, sociological and other patterns of the TTWA (Travel-to-work-area) Hodonín in Czechia. Various data sources such as census, land register, transport data or data reflecting the shopping behaviour, were employed to meet this purpose. On this multi-graph structure we run a modified SEIR model of the COVID-19 dynamics. The parameters of the model are calibrated on data from the outbreak in the Czech Republic in the period March to June 2020. The simplest type of contact tracing follows just the family, the second tracing version tracks the family and all the work contacts, the third type finds all contacts with the family, work contacts and friends (leisure activities). The last one is a complete (digital) tracing capable of recalling any and all contacts. We evaluate the performance of these contact tracing strategies in four different environments. First, we consider an environment without any contact restrictions (benchmark); second with strict contact restriction (replicating the stringent non-pharmaceutical interventions employed in Czechia in the spring 2020); third environment, where the measures were substantially relaxed, and, finally an environment with weak contact restrictions and superspreader events (replicating the situation in Czechia in the summer 2020). FINDINGS: There are four main findings in our paper. 1. In general, local closures are more effective than any type of tracing. 2. In an environment with strict contact restrictions there are only small differences among the four contact tracing strategies. 3. In an environment with relaxed contact restrictions the effectiveness of the tracing strategies differs substantially. 4. In the presence of superspreader events only complete contact tracing can stop the epidemic. INTERPRETATION: In situations, where many other non-pharmaceutical interventions are in place, the specific extent of contact tracing may not have a large influence on their effectiveness. In a more relaxed setting with few contact restrictions and larger events the effectiveness of contact tracing depends heavily on their extent.

Centre for Mathematical Biology Institute of Mathematics Faculty of Science University of South Bohemia Branišovská 1760 37005 České Budějovice Czech Republic

Centre for Modelling of Biological and Social Processes Na Břehu 497 15 190 00 Praha 9 Czech Republic

CERGE EI Politických vězňů 7 11121 Praha 1 Czech Republic

Czech Academy of Sciences Biology Centre Institute of Entomology Department of Ecology Branišovská 31 37005 České Budějovice Czech Republic

Department of Biochemistry Cell and Molecular Biology 3rd Faculty of Medicine Charles University Ruská 87 100 00 Praha 10 Czech Republic

Department of Criminology and Mitchell Centre for Social Network Analysis School of Social Sciences University of Manchester Oxford Rd Manchester UK

Department of Mathematics University of Zagreb Bijenička 30 10000 Zagreb Croatia

New Media Studies Faculty of Arts Charles University Na Pří kopě 29 110 00 Praha 1 Czech Republic

The Czech Academy of Sciences Institute of Computer Science Pod Vodárenskou věží 2 18200 Praha 8 Czech Republic

The Czech Academy of Sciences Institute of Information Theory and Automation Pod Vodárenskou věží 4 18200 Praha 8 Czech Republic

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