Herd immunity will be reached sooner than we thought

Even with the best intentions, the SARS-CoV-2 virus is spreading through the South Africa population at a scary pace.  The NMG model fit to confirmed cases to 10 May measures viral propagation at a reproduction number of 2.26.  The latest model can be found at https://nmg-covid-19.sctechnology.co.za/

At this rate according to the logic underlying the model, some 90% of the population will become infected with the virus before the virus stops spreading.  The virus stops spreading and dies out in a population when enough lives have immunity (having developed this immunity from an earlier infection).

The logic of the SEIR epistemological models assumes that the virus will move through a population where all individuals are equally susceptible to infection.  In practice the virus will be constrained in its movements by the social contact networks that exist in the population.  The virus will infect the highly connected individuals first, and then the viral propagation will slow down when less connected individuals are left in the susceptible population.

This slowing down of the viral propagation has now been built into the NMG model so as not to overstate the numbers that are forecast to eventually become infected.  A herd immunity parameter must be entered that represents the percentage of the population that is infected when the virus stops spreading in the population (when the reproduction number is below 1.00).  The model transmission rate parameter is reduced in stages until this herd immunity parameter is attained to reflect this slowing down of the viral propagation.

A herd immunity parameter of 30% has been used as the default in the NMG model based on historical experience with past influenza pandemics (most notably the 1918 and 1957 pandemics) where around 30% of the population seem to have become infected. This has reduced the total COVID-19 deaths forecast by the NMG to model to just over 60 000 which is within the range of 48 300 and 88 000 given by the four scenarios in the Actuarial Society model.  

A comparison of this forecast for COVID-19 deaths and all-causes deaths is given below:

The all-causes deaths in 2020 continue to track below the numbers in 2019 due to the lockdown constraints in South Africa.  The peak in COVID-19 deaths is expected to arrive in August.  Let's hope that our hospitals will be ready to treat these patients.

12 May 2020



Comments

Post a Comment

Popular posts from this blog

Update on COVID 19 tracking in SA

Image
An analysis of the weekly deaths from natural causes as reported by the SA Medical Research Council (SAMRC) shows evidence of elevated excess deaths over the winter months of 2022. The graph below shows the weekly deaths reported by SAMRC in excess of the average weekly deaths reported between 1 January 2020 and 15 March 2020.  The COVID-19 hospital deaths as reported by the National Institute for Communicable Diseases (NICD) would appear to follow a similar pattern to that shown by the SAMRC excess deaths but with a more muted 5th wave. Anecdotally, the number of COVID-19 cases appear to be on the rise in South Africa although COVID-19 cases are no longer reported on a regular basis (outside of a hospital setting).  The above graphs show that the COVID-19 disease severity is certainly not near the levels experienced in the first three waves. 4 November 2022
Read more

Time to consider the 4th wave?

Image
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...
Read more

Is the third wave less deadly?

Image
KwaZulu Natal has overtaken both Gauteng and the Western Cape in the number of reported COVID-19 deaths. However, the deaths in KwaZulu Natal are still well off the peak experienced in the second wave.  The modeling done by NMG shows that the reported COVID-19 deaths are still lagging way behind what is projected for the third wave based on the reported infections. This would indicate that the likelihood of dying if infected in the third wave has not been as high as for the first two waves. This may be due to greater levels of prior exposure to the virus of those infected due to either vaccination or prior infection. It could also be the result of more effective medical treatment. 19 August 2021
Read more