Today I’m taking a look at why the United States Covid-19 death curve is following neither the same trajectory as most European countries, nor that of most South American countries. It seems to have the same initial peak as Italy, but then a post-peak surge that matches Brazil:

So, what’s going on? Well, first it’s worth understanding why Italy and Brazil are on such different journeys.

The figure below shows the 4 classic curves for Temperate and Tropical climates in the Northern and Southern Hemispheres.

Italy obviously has a Northern Temperate climate, and despite the efforts of the WHO and governments to flatten the curve through lockdowns and mask wearing, Covid-19 here has followed the same trajectory as for all European influenza and coronavirus epidemics throughout history.

Instead, Brazil is a Southern Tropical climate and demonstrates the expected, flatter curve. Sorry the line is so ragged, but deaths aren’t reported daily, so there is some bounce :

Now, you can probably guess what is happening. The US is not experiencing a second wave, it is simply so large that it contains both temperate and tropical climates.

In the first half of the epidemic, Covid-19 in the US was confined to the North East (NY, MA, MI, PA, etc) and was therefore a Temperate Epidemic. In the second half the southern and western states were affected (FL, TX, WA, CA, AZ, MO, etc) and it became a Tropical Epidemic :

To make this clearer, I have split the US into Temperate and Tropical zones and clustered them into separate graphs :

Pretty neat huh? It certainly resolves some questions I’ve been asking about how the US is progressing, but it also starts to make us think twice about some of the advice circulating on social media :

I may have even used a picture like this to convince my parents to stay home and stay safe, but like much of what you read online, this is logical, but not necessary correct. Notice that the green curve looks like that of the North Eastern US cluster and the red one looks like the Southern US cluster one?

So, is this the result of social distancing, or is it just because the virus behaves significantly differently in hot humid climates than cool dry ones?

As usual, I don’t think the answer is black and white. I personally don’t believe that the spread of the virus is unaffected by social response, and I would always advise the vulnerable to take precautions, however I do think that there is some evidence to show that the epidemic ‘runs its course’ in each geography, and the shape of the curve may have as much to do with the way viruses spread in temperate vs tropical climates than whether or not a country locks down.

As usual the pretty charts came from my Observable Notebook that you can access on my Covid-19 page and modify / fork here.