Successful contact tracing systems for COVID-19 rely on effective quarantine and isolation

James, Alex; Plank, Michael J; Binny, Rachelle N; Lustig, Audrey; Steyn, Nicholas; Hendy, Shaun; Nesdale, Annette; Verrall, Ayesha

Background. Test, trace and isolate are the three crucial components of the response to COVID-19 identified by the World Health Organisation. Mathematical models of contact tracing often over-simplify the ability of traced contacts to quarantine or isolate. Method. We use an age-structured branching process model of individual disease transmission combined with a detailed model of symptom onset, testing, contact quarantine and case isolation to model each aspect of the test, trace, isolate strategy. We estimated the effective reproduction number under a range of scenarios to understand the importance of each aspect of the system. Findings. People's ability to quarantine and isolate effectively is a crucial component of a successful contact tracing system. 80% of cases need to be quarantined or isolated within 4 days of quarantine or isolation of index case to be confident the contact tracing system is effective. Interpretation. Provision of universal support systems to enable people to quarantine and isolate effectively, coupled with investment in trained public health professionals to undertake contact tracing, are crucial to success. We predict that a high-quality, rapid contact tracing system with strong support structures in place, combined with moderate social distancing measures, is required to contain the spread of COVID-19. Funding. This work was funded by the Ministry of Business, Innovation and Employment, New Zealand and Te Punaha Matatini, the NZ Centre of Research Excellence for Complex Systems.