SARS-CoV-2 Omicron disease burden in Australia following border reopening: a modelling analysis
Authors: Milne George J , Carrivick Julian 
Published: March 2022

Background

Countries with high COVID-19 vaccination rates have seen the SARS-CoV-2 Omicron variant result in rapidly increasing case numbers. This study evaluated the impact on the health system which may occur following introduction of the Omicron variant into Western Australia following state border reopening. We aimed to understand the effect of high vaccine coverage levels on the population health burden in the context of lower vaccine effectiveness against the Omicron variant, the impact of a third dose booster regime, and ongoing waning of vaccine-induced immunity. Originally scheduled for 5thFebruary 2022, the Western Australian border was opened on 3rdMarch 2022, we also aimed to determine the impact of delaying border reopening on the COVID-19 health burden and whether the West Australian health system would be able to manage the resulting peak demand.

Methods

An agent-based model was used to evaluate changes in the COVID-19 health burden resulting from different border openings, at monthly intervals. We assumed immunity was derived from vaccination with the BNT162b2 Pfizer BioNTech vaccine and waned at observed rates from the UK. The model was calibrated against outbreaks in two other Australian states, Queensland and South Australia, both of which were in a similar situation to Western Australia with negligible COVID-19 transmission prior to Omicron’s introduction. Age-specific infections generated by the model, together with recent UK data, permitted resulting outbreak health burden to be quantified, in particular peak ICU demand.

Results

Overall population immunity in Western Australia is shown to peak and then plateau for a period of 5 months, between February and June 2022, resulting in a similar health burden if the border is reopened prior to June 2022. For an opening date of 5thMarch 2022, hospitalisations are predicated to peak at 510 beds, 51 of which will be in ICU, with a total of 383 deaths. If the border reopened on 5thJune 2022, hospitalisations are expected to peak with 750 beds required, 75 of which would be in ICU, and a total of 478 deaths. With a total surge capacity of 52 fully staffed ICU beds, West Australian hospitals are predicted to have adequate ICU capacity for future COVID-19 demands if border reopening occurs prior to May 2022.

Conclusions

Our results show that with extremely high SARS-CoV-2 vaccination rates in Western Australian, and documented vaccine-induced vaccine waning rates, the overall population immunity in Western Australia will be at its highest in the period of February 2022 to June 2022. Opening the Western Australian border prior to the end this period will result in the lowest health burden in comparison to opening in June 2022 or later. With a border reopening of 3rdMarch 2022 announced by the Western Australian government, our data for a 5thMarch 2022 opening date may be used to predict the progression of this resulting outbreak. These data show expected peak demand of 510 hospital beds, 51 of which will be in ICU, with a total of 383 deaths. With a surge capacity of 52 ICU beds, it is expected that the Western Australian hospital system will be able to handle the additional load during the peak of the wave.