August 24th, 2020
It appears that Victoria (and Australia) are still on track to get down to single digit New Cases per day by mid-September as the second chart below suggested on 26 July 2020. Admittedly, it looked shaky a few times. Some residents were non-compliant with restrictions during the two periods shown with light blue arrows below. And the rate of reduction in the infection ratio (shown by R8, the 8 day mean of the daily R calculated from the actual Australian cases) was slower in this second wave (the purple trend-line) than it was during the first wave (the blue trend-line).
The grey curve shows the number of New Cases a day predicted by the epidemic model using the inputs in the grey box to the left of the chart. The curve is seeded from the actual Australian cases for the 8 days prior to the start date of the curve (set at 28/6/2020) and the start date, the initial R, the rate of fall of R, the i , and the stop date for the fall of R (the infection ratio). The best match to the curve of the actual Australian cases was as shown. R is best set set to fall by 0.015 each day from 1.55 between the start date and the stop date (30/8/2020) after which it remains unchanged. R therefore falls from 1.55 to 0.62 during this period. The actual Australian new cases per day is shown in green, and they appear to be following and well matched to the grey curve.
The chart below shows the actual Australian new cases a day on 26 July 2020 in green. Various values had been input into the grey box on the left of the chart in order generate the grey curve to best match the actual Australian new cases up to that time. R increases and i (the number of days a case remains infectious before the cases is removed from the population by isolation) decreases the slope of the grey curve at any point. Thus, only one value of R/i will best match the slope of the curve of actual Australian cases. The value of 6 for i best matched the first wave of Australian cases. R had fallen steadily during the first wave, and so was expected to fall steadily during this second wave. It should be noted that i is not the number of days a case is actually or medically infectious. It is the number of days that a cases can infect another in practice because it is the number of days after infection until the case is removed from infecting the population by being isolated or medically managed.
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Dr Michael Cole FRACP LLB