Figure 1.

The Number of Cumulative Confirmed Cases Was Calibrated to a Bi-logistic Function, Which Was Used to Model Biologic Patterns with Two Growth Phases

The details of the bi-logistic function have been documented previously by Meyer et al.² The fitting of the bi-logistic function with six parameters was conducted on https://logletlab.com/. The parameters K₁ (1,491) and K₂ (18,350) represent the asymptotic values that bound the function and therefore specify the level at which the epidemic saturates; t_m1 (29 March) and t_m2 (31 July) represent the midpoint of the growth of the two epidemics and hence the peak of the two outbreaks; Δt₁ (23 days) and Δt₂ (43 days) are the time intervals required for the epidemics to grow from 10% to 90% of the saturation level, as defined by the bi-logarithmic function; in contrast, we defined the duration of the epidemics ΔT₁ (48 days) and ΔT₂ (88 days) as the length of time required for the epidemic to grow from 1% to 99% of the saturation level. The first lockdown period was between 16 March and 25 May 2020 (the State of Emergency was declared on 16 March and Stage 2 restrictions were in place between 25 March and 25 May). The second lockdown period was between 9 July and 13 September (9 July to 20 August, stage 3 for all Victoria; 21 August to 13 September, stage 4 for metro Melbourne and stage 3 for regional Victoria). Compulsory use of face masks in public spaces was implemented on 23 July. The first step of the COVID-19 Roadmap for reopening was initiated on 14 September with most of the stage 4 restrictions remaining.