Time to consider the 4th wave?

We have updated the NMG model to consider various scenarios for the 4th wave of COVID-19 infections for South Africa. You can access the model at https://nmg-covid-19.sctechnology.co.za/

It will take a sustained viral propagation above a threshold speed to generate a noticeable 4th wave. The first 4th wave scenario is displayed below assuming a viral propagation speed of Rt = 3.0 for a period of 100 days starting on 15 October 2021. 


The second scenario shows the impact of a viral propagation speed of Rt = 4.0 for the 4th wave. It is important to note that to reach Rt = 4.0 in the current South African population would require a viral strain with a basic reproduction number of over 12 (some four times the speed of the original SARS-CoV-2 virus). This calculation is based on the NMG model, where some 67.5% of the South African population is assumed to have built up immunity to reinfection due to infection over the first three waves. 


A material 4th wave could be caused by a viral strain with a slower speed of propagation but one that is less susceptible to immunity. The third scenario shows the impact of a viral propagation speed of Rt = 3.0 where some 5% of the population with immunity is susceptible to reinfection.


It is then fairly easy to generate a scenario that shows a 4th wave that is more devastating than the first three waves in South Africa. The fourth scenario below shows the impact of a viral propagation speed of Rt = 3.0  where some 10% of the population with immunity is susceptible to reinfection.


Every time someone is infected with the SARS-CoV-2 virus there is an opportunity to generate a variant that could be more destructive than the current ones. These destructive variants are, however, very rare and are only generated when a large number of infections occur like what happened in India with the Delta variant.

30 September 2021

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