The Western Cape should reach its peak in daily new infections tomorrow!

The latest prediction for the COVID-19 death toll in the Western Cape was revised upwards to 9300 at a presentation by the SA COVID-19 Modelling Consortium on 28 May 2020. I used this prediction to calibrate the NMG model for the pandemic in the Western Cape, and came to the starting conclusion that we should be reaching the peak in daily infections tomorrow (10 June).  This is startling because if we don't it means that the 9300 prediction is an under-estimate (at least according to the NMG model).

Let's start with the forecast for the COVID-19 deaths.  What we know from the reported COVID-19 deaths in the Western Cape is that the virus is moving through the population at a speed represented by a reproduction number of 2.60 (or 2.6 new infections for every infected person).  If we fix the total deaths at 9300, we can predict that the new daily reported COVID-19 deaths will reach a peak on 1 July when 185 deaths will be reported.



The number of hospital beds can be inferred from the COVID-19 deaths using a hospital admission rate of 5 times the Infection Fatality Ratio.  This gives a peak in the number of hospital beds required in the Western Cape of just below 5 500 on 25 June.  The ratio of 5 times is aligned with the experience in the public sector where the latest data reports a moving average of 100 hospital admissions and 20 deaths per day.  The ratio may well be higher in the private sector.


And then for the startling conclusion.  If I work backwards using an Infection Fatality Ratio of 0.60% adjusted for the age-distribution of deaths reported in the Western Cape, I conclude that 33% of the Western Cape population will need to be infected to generate the 9 300 deaths.  Modelling the infections using the NMG model and data reported to 7 June, the daily infections reach a peak on 10 June.


According to the NMG model only 5% of the total infections are reported and any patterns in the actual reported infections can then materially distort the situation.  It is for this reason that we switched to calibrating the NMG model using reported deaths rather than reported infections some time ago.

The NMG model also assumes that 31% of those infected will be asymptomatic and at no risk of either death or hospitalisation (for those doing a sanity check on the numbers!). 

The NMG model uses a herd immunity parameter of 20% for South Africa with an upper bound for the parameter of 40%.  The experience in the Western Cape would indicate that we are already well past a 20% value for the herd immunity parameter and in danger of breaching the 40% level unless we do indeed reach the peak in daily infections in the next day or so.

Startling news indeed!

9 June 2020

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