  An outbreak of the Delta (B.1.617.2) variant of SARS-CoV-2 that began around
mid-June 2021 in Sydney, Australia, quickly developed into a nation-wide
epidemic. The ongoing epidemic is of major concern as the Delta variant is more
infectious than previous variants that circulated in Australia in 2020. Using a
re-calibrated agent-based model, we explored a feasible range of
non-pharmaceutical interventions, including case isolation, home quarantine,
school closures, and stay-at-home restrictions (i.e., "social distancing"). Our
modelling indicated that the levels of reduced interactions in workplaces and
across communities attained in Sydney and other parts of the nation were
inadequate for controlling the outbreak. A counter-factual analysis suggested
that if 70% of the population followed tight stay-at-home restrictions, then at
least 45 days would have been needed for new daily cases to fall from their
peak to below ten per day. Our model predicted that, under a progressive
vaccination rollout, if 40-50% of the Australian population follow stay-at-home
restrictions, the incidence will peak by mid-October 2021: the peak in
incidence across the nation was indeed observed in mid-October. We also
quantified an expected burden on the healthcare system and potential fatalities
across Australia.
