Successful contact tracing systems for COVID-19 rely on effective quarantine and isolation
Authors: James A. , Plank M.J. , Hendy S. , Binny R. , Lustig A. , Steyn N. , Nesdale A , Verrall A 
Published: June 2020

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.

Evidence before this study

Existing models of contact tracing concentrate on the time taken to trace contacts and the proportion of contacts who are traced, often focussing on the differences between manual and digital tracing. They often over-simplify the quarantine and isolation aspect of contact tracing. For example, some models assume that isolation and quarantine are 100% effective in preventing further transmission, while others treat tracing coverage and isolation effectiveness as interchangeable. Numerous performance indicators have been used to measure the effectiveness of a contact tracing system. However, it is frequently not known how reliably these indicators measure the reduction in in onward transmission under a range of unknown parameters.

Added value of this study

We explicitly model the effectiveness of contact quarantine and case isolation in reducing onward transmission and show that these are not equivalent to tracing coverage. For example, isolating 50% of contacts with 100% effectiveness gives a much larger reduction in onward transmission than isolating all contacts but with only 50% effectiveness. We show that, although tracing speed is important, without effective isolation and quarantine it is a waste of effort. We show that seemingly straightforward indicators of contact tracing effectiveness are unreliable when the effectiveness of isolation is not guaranteed. We propose an indicator based on the time between quarantine or isolation of an index case and quarantine or isolation of secondary cases that is more robust to unknowns.

Implications of all the available evidence

Establishing support systems to enable individuals to quarantine and isolate effectively is equally important as implementing a fast and efficient contact tracing system. Effective contact tracing requires a skilled, professional workforce that can trace downstream contacts of a positive case, as well as upstream contacts to determine the source of infection and provide the high quality data needed. Over-reliance on digital contact tracing solutions or the use of untrained contact tracing staff are likely to lead to less favourable outcomes.