While contact tracing alone is unlikely to contain the spread of COVID-19, it may allow population-wide social distancing measures to be relaxed. There is a need for robust ways to measure the effectiveness of contact tracing in reducing the spread of COVID-19. The effective reproduction number measures the current transmission rate of the virus and the aim of many public health interventions is to reduce the effective reproduction number. However, it is difficult to disentangle the effects of multiple simultaneous interventions, such as case-isolation, contact tracing, and population-wide restrictions, using real-time estimates of the effective reproduction number alone. We therefore need reliable operational indicators to measure the impact of contact tracing.
We develop a model that allows us to investigate the importance of these factors in reducing the effective reproduction number. We explicitly model the effectiveness of contact quarantine and case isolation as independent variables, and allow quarantine of contacts who are currently not symptomatic to be less effective than isolation of confirmed cases. This is a realistic model assumption that reflects greater likelihood of behaviour change, increased levels of support, or greater mandatory regulation for isolation of cases following a positive test result. This allows us to separately investigate the impact of increasing effectiveness of quarantine and isolation. We use a model calibrated using data on time from symptom onset to isolation and test result of COVID-19 cases in New Zealand, where community transmission of the virus was eliminated in June 2020.
We show that the reduction in onward transmission during quarantine and isolation has a bigger effect than tracing coverage on the reproduction number. We also show that intuitively reasonable contact tracing performance indicators, such as the proportion of contacts quarantined before symptom onset, are often not well correlated with the reproduction number.
We conclude that provision of support systems to enable people to quarantine and isolate effectively is crucial to the success of contact tracing.
[Full paper] (https://doi.org/10.1371/journal.pone.0252499)