Modelling the second wave

I spent the last week updating the NMG model to forecast a series of second waves of COVID-19 infections for South Africa. The modelling is made more complicated as some provinces have not yet seen the back of the first wave and at least one province (the Eastern Cape) is showing clear signs of a second wave.

The updated model has a default (or best estimate) projection that shows 34% of the South African being infected with SARS-CoV-2 by 31 May 2021. This scenario assumes a second wave of infections will materialise in only one province, namely the Eastern Cape. The other scenarios show the impact of a modest second wave in most provinces (45% infected) and a severe second wave (60% infected).


The forecast reported and unreported COVID-19 deaths for these three scenarios are 50000, 63000 and 87000 respectively.

I also needed a second (and hopefully more reliable) data point for monitoring the presence of a second wave in the various provinces.  A second fit has now been added to the model using COVID-19 hospital admissions reported to the National Institute for Communicable Diseases (NICD). The reporting on COVID-19 hospital admissions has become much more stable over the last month and currently covers 99% of public and 100% of private hospital admissions.


The historical fit of the NMG model with reported COVID-19 hospital admissions is not as accurate as that for the reported COVID-19 deaths given that the primary data point used to calibrate the model was reported deaths. Going forward, the model will be calibrated using the reported COVID-19 hospital admissions as these are expected to be a more reliable indicator of a second wave of infections.

It is important to state that my own view is that a material second wave of infections is unlikely for South Africa as a whole given the high levels of seroprevalence already in the population. There may however be sub-populations with low seroprevalence where the risks of a second wave is higher.

The weekly reports on COVID-19 hospital admissions by the NICD are a good resource for monitoring the emergence of second waves in sub-populations of South Africa. The latest report identifies a resurgence of COVID-19 hospital admissions in the Mandela Bay Metro and is available at https://www.nicd.ac.za/diseases-a-z-index/covid-19/surveillance-reports/weekly-sentinel-hospital-surveillance-datcov-update/.


21 November 2020



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