Please register here: https://zoom.us/meeting/register/tJMrdOCsrz4jH9SXpn1gJLgslrwytZmiaOZv . With confirmed cases of COVID-19 declining in many countries, lockdown measures are gradually being lifted. However, even if most social distancing measures are continued, other public health measures will be needed to control the epidemic. Contact tracing either via conventional methods or via mobile app technology is central to control strategies during de-escalation of social distancing. In my talk, I will present results on the impact of timeliness and completeness on the effectiveness of contact tracing strategies (CTS). To analyze the impact of various steps of a CTS, we developed a stochastic mathematical model with explicit time delays between time of infection, symptom onset, diagnosis by testing, and isolation. The model also includes tracing of close contacts (e.g. household members) and casual contacts with different delays and coverages. We computed effective reproduction numbers of a CTS for a population with social distancing measures and various scenarios for testing, isolation of index cases, and tracing and quarantine of their contacts. We quantified the percent onward transmissions per diagnosed index case that can be prevented by CTS depending on timeliness and completeness of isolation and tracing.