Bell-curve skewed in USA – Why ?

Back in March, Professor Sunetra Gupta, Professor in theoretical epidemiology at the University of Oxford published a paper which speculated that as much as 50% of the UK population may already have been infected with covid-19, and the true Infection Fatality Rate may be as low as 0.1%.

Observing the very similar patterns of the epidemic across countries around the world convinced Professor Gupta that it is hidden community immunity, more than lockdowns or government interventions, that offer the best explanation of the Covid-19 progression:

“In almost every context we’ve seen the epidemic grow, turn around and die away — almost like clockwork. Different countries have had different lockdown policies, and yet what we’ve observed is almost a uniform pattern of behaviour which is highly consistent with the SIR model. To me that suggests that much of the driving force here was due to the build-up of immunity. Click for full Unherd interview.

This hypothesis is clearly demonstrated in the following graphs from the web site InProportion2 , which show similar shaped bell curves of the first wave epidemics of six nations, but shows US covid-19 deaths have risen, peaked and fallen very differently from those of European countries.

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Why is the US different?

In the larger countries of Western Europe, the scale of the death toll has varies significantly, but the shape of the mortality curve has not. In most of Western Europe, covid-19 deaths have risen and fallen around a single peak in a familiar bell-shaped curve.

As shown in the above image, covid-19 mortality curves for Europe are very similar to the curve predicted by the so-called ‘SIR Model’. SIR, developed by Scottish scientists Kermack and McKendrick in the 1920s and 1930s, is a mathematical model used to predict the path of a number of different infectious diseases.  Notice how the curve rises slightly more steeply than it declines in both the SIR model and the covid data. The similarity between this decades-old mathematical model and 2020 covid mortality curves is striking. This observation suggests a natural process and not one resulting mainly from human interventions, such as national or local lockdowns.

However, the US covid-19 mortality curve does not follow the pattern of these European countries and by extension the SIR model. Why is the US different? Many have observed that the Southern US is having a very different covid-19 experience from the North. A reasonable hypothesis is that the US mortality curve is a composite of other curves, one overlaying the other to give the overall curve. A possible explanation for the varying forms of the underlying curves could be related to regional and climactic differences.

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The InProportion article contains far more extensive analysis and graphs, and for those interested, click here. The main conclusions are :

  • Covid deaths have risen and fallen over time in a way which correlates with climate/latitude zone rather than with the scope and timing of interventions.
  • The effect of quarantines and lockdowns in the US on the rise and fall of covid deaths appear insignificant compared to the effect of climate and latitude.

The other significant conclusion of this data is that Professor Gupta is correct in her hypothesis. Whether or not there is a second wave of deaths this winter remains to be seen. To date, cases are surging in Europe and America, but deaths remain very low compared with the first wave. Time will tell.

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