Modelling herd immunity requirements in Queensland: impact of vaccination hesitancy and variants of SARS-CoV-2

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Long-term control of SARS-CoV-2 outbreaks depend on widespread coverage of effective vaccines. In Australia, the vaccine rollout achieved >90% two-dose coverage of the adult population. However, during August 2020 to August 2021, hesitancy rates fluctuated dramatically. This left the question whether settings with low naturally-derived immunity like Queensland, where <0.005% of the population is known to have been infected in 2020, could have achieved herd immunity against variants of concern that emerged in 2021. To address this, we used an agent-based model Covasim. We simulated outbreak scenarios (with either Alpha or Delta), and assumed ongoing interventions (testing, tracing, isolation, and quarantine). We modeled the rollout using two approaches, each affording us different levels of realism. Hesitancy was modeled using survey data. We found that with a vaccine effectiveness against infection of 80% it may have been possible to control outbreaks of Alpha, but not Delta. With 90% effectiveness, control of Delta outbreaks may have been possible but only briefly before waning immunity rendered the population susceptible. Even with uncontrolled outbreaks, we estimated that a decrease in hesitancy from 20% to 14% would reduce the number of infections, hospitalizations, and deaths by over 30%. Overall, we demonstrate that while herd immunity may not be attainable, modest reductions in hesitancy and increases in vaccine uptake have the potential to greatly improve health outcomes.