Quantifying the impact of contact tracing interview prioritisation strategies on disease transmission: A modelling study

Contact tracing is an important public health measure used to reduce transmission of infectious diseases. Contact tracers typically conduct telephone interviews with cases to identify contacts and direct them to quarantine, with the aim of preventing onward transmission. However, in situations where caseloads exceed the capacity of the public health system, timely interviews may not be feasible for all cases. Here we present a modelling framework for assessing the impact of different case interview prioritisation strategies on disease transmission. Our model is based on Australian contact tracing procedures and informed by contact tracing data on COVID-19 cases notified in Australia from 2020 to 2021. Our results demonstrate that last-in-first-out strategies (where cases with the most recent swab or notification dates are interviewed first) are more effective at reducing transmission than first-in-first-out strategies (where cases with the oldest swab or notification dates are interviewed first) or strategies with no explicit prioritisation. To maximise the public health benefit from a given case interview capacity, public health practitioners may consider our findings when designing case interview prioritisation protocols for outbreak response.Author summary: Contact tracing can be an effective strategy for controlling infectious diseases, including COVID-19. Contacts of cases, identified through case interviews, may be directed to quarantine before they become infectious, interrupting chains of disease transmission. Contact tracing is limited by available resources, including the number of case interviews that can be conducted per day. When contact tracing systems have exceeded workforce capacity, it may not be possible to interview all cases in a timely manner. Hence, an approach for prioritising cases for interview is necessary to maximise transmission reduction within the available interview capacity. We use a simulation modelling framework to quantify the impact of different interview prioritisation strategies on the transmission of SARS-CoV-2. We find that prioritising the most recently identified cases for contact tracing increases the impact of test-trace-isolate-quarantine responses, since late case finding results in diminishing returns on transmission reduction. Contact tracing management systems should be designed to implement a prioritisation strategy for case interviews that achieves the greatest reduction in transmission, among other system objectives, for a given interview capacity.