So after all this modelling, are we any nearer forecasting the 'first wave' impact of COVID-19 on South Africa?

There is so much we don't know about SARS-CoV-2 and how it moves through and infects a population.  But what we do know is that there are many examples of how the virus has impacted populations in Europe and North America that we can surely learn from.  Maybe these learning can help us with our 'first wave' forecasting for South Africa?

I spent the weekend fitting the NMG model to five international populations (three countries, one epicenter region and one epicenter city) that are all at the end of their 'first wave' SARS-CoV-2 experience.  The graph below shows the fit for the reported deaths in Italy, one of the countries that I selected:


The 'first wave' of reported deaths for all five populations follow a similar pattern of a steep incline to a sharp peak followed by a less steep decline.  In all five populations there is no sign (yet) of a 'second wave' in reported deaths.

Leaving aside the 'second wave' theories, we in South Africa must survive our 'first wave' that is fast approaching.  I was then interested in the insights that the NMG model could reveal in terms of the percentage of the population infected in these international first waves.  This would hopefully help with the calibration of the 'herd immunity parameter' for the NMG model for South Africa.

The table below sets out three key features of the 'first wave' experiences in the five international populations as derived using the NMG model:


 A test for reasonableness can be obtained from sample antibody testing that has been done in some of the populations.  I could find studies for Spain and New York City that estimated the percentage of the population that had had COVID-19 at 5% (for Spain) and 19.9% (for New York City).  Based on this, I have concluded that the estimates from the NMG model might be on the conservative side.

The big leap, off course, is whether the viral spread in these populations will be anything like what we may experience in South Africa. In the latest NMG model, we have opted for a default herd immunity parameter of 15% with a range of 5% to 35%. 


25 May 2020



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