Taking a closer look at the 'first wave' infections in the Western Cape

The Western Cape is now firmly on the lower slopes of the COVID-19 'first wave' of infections, and soon we will be listening to the provincial leadership predicting when we might be over the peak much like the Governor of New York did in his then daily press conferences.

Last weekend I analysed the infection wave in New York City and the waves for four populations in Europe. This week it is time to compare the early indications of the infection wave in the Western Cape to see whether this is following a similar pattern.  The graph below shows the fit of the NMG model to the new daily deaths reported for the Western Cape:


There are two differences with the Western Cape infection wave.  The first is that we are moving up the slope at a slower rate.  The reproduction number for the Western Cape is measured at 2.60 whereby each infected life generates 2.6 new infections.  The upslope for all five international populations was measured at a reproduction number of 4.50.  The reason might be that the virus is moving through a population that was locked down earlier in South Africa than was the case in the other populations.

This may not be a good thing. In the international populations, lockdown-type interventions and behaviour change by a frightened population could slow the viral propagation and ensure that the 'first wave' peaks were reached sooner. In South Africa, there is little else that can be done to slow the virus down.

The second difference with the Western Cape infection wave is that, using the NMG model, we estimate that already more than 5% of the population is infected.  Our fitting of the international infection waves is such that the peak is reached when roughly 50% of the total numbers infected is reached.  We are then already at the point where 10% of the Western Cape population will be infected in the first wave. Our modelling showed that the total numbers infected in the 'first wave' of infections in Italy, Lombardy and Spain were well below 10%.

The three infection waves modeled for the Western Cape in the earlier graph use herd immunity parameters of 10%, 20% and 35% respectively.  When looking at the international experience last week, I had concluded that a default herd immunity parameter of 15% might be conservative for the South African model.  After this analysis, I am not sure that this is the case.


29 May 2020

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