Border quarantine, vaccination and public health measures to mitigate the impact of COVID-19 importations in Australia: a modelling study

We developed a flexible infectious disease model framework that combines a detailed individual-based model of arrival pathways (quarantine model) and an individual-based model of the arrivals environment (community model) to inform border risk assessments. The work was motivated by Australia's desire to safely increase international arrival volumes, which had been heavily constrained since early 2020 as a result of the COVID-19 pandemic. These analyses supported decisions on quarantine and border policy in the context of the Australian government's national reopening plan in late 2021. The quarantine model provides a detailed representation of transmission within quarantine and time-varying infectiousness and test sensitivity within individuals, to characterize the likelihood and infectiousness of breaches from quarantine. The community model subsequently captures the impact on these infectious individuals in the presence of varying vaccination coverage, arrival volumes, public health and social measures (PHSMs) and test-trace-isolate-quarantine system effectiveness in the Australian context. Our results showed that high vaccination coverage would be required to safely reopen with support from ongoing PHSMs, and quarantine pathways have minimal impact on infection dynamics in the presence of existing local transmission. The modelling pipeline we present can be flexibly adapted to a range of scenarios, and thus provides a useful framework for generating timely risk assessments in the event of future pandemics.