Rice on the Chessboard

3Blue1Brown have a good video explaining the math and mechanics of how a virus spreads in a population. Watch it here.1

The first take-away from the video is that most people completely underestimate the effect and power of exponential growth. You can easily test this with your friends. Ask them to imagine a chess board, with its 64 fields. Now, you put one grain of rice on the first field, and for the sake of argument we assume that one grain weighs one tenths of a gram. If you continue by doubling the number of grains on each consecutive field – how many kilograms of rice will be on the 64th field? It’s 1.8 million million tons (1.8 * 1012 tons). That’s pretty far off of most peoples’ estimates.2 Many think they estimate high if they say 100kg. Starting from the idea of just one tenth of a gram, they cannot wrap their mind around the power of 264 (pun intended).

More on the topic, ie. the spreading of a virus, is another point in the video, depicted in the picture above. See the term E * p? The meaning of the E and p are given in the figure. A reduction of exposure (E, reducing contact, eg. avoiding to meet people) multiplied by probability (p, reducing transfer probability, eg. washing your hands) from 0.15 to 0.05 – ie. to a third – reduces the number of new cases within 61 days from over 105 million people to just over 400,000, when starting from 21,000 existing cases. That’s what “social distancing”, to use this new stupid buzzword, is all about.

Germany’s Angela Merkel said yesterday that her experts expect that up to two thirds of their population could get infected over the coming months, or even years. A US expert (Amesh Adalja3) estimates that 20% to 50% of the people will become infected. Remember, a vaccine is 12 to 18 months away, and anti-viral treatments are only in first clinical trials right now.4 In most countries, no-one expects anymore to be able to keep the new corona virus out. It’s all about avoiding to have too many severe cases at once for the health care system to be able to handle. “Social distancing” for “flattening the curve”. We’ll have to learn to live with SARS-CoV-2, as well with any “novel” corona virus in the future.5

Mauritius has implemented strict immigration rules for anyone entering the island. The health screening at the airport, which is standard procedure anyway, has been intensified and systematised. The official public celebrations of today’s national independence day have been cancelled. Sound measures, but it’s not a question of if, but when we’ll have cases on the island as well. It only takes one tourist, say from Switzerland – which is currently not on the hot list of countries –, carrying the virus without showing any symptoms, and we’ll have a hotel full of Covid-19 cases quickly.

The Guardian has this interesting article, to give some more perspective: What Ebola taught me about coronavirus: panic will get us nowhere:

If there is one lesson I learned from my research on Ebola in Sierra Leone, it is this: take care but don’t panic about the virus and lose sight of the bigger picture. I hope that in responding to coronavirus, we as individuals – and our institutions – can learn something from those who have been through this before.

It’s hard to shed the impression that now, when “developed” countries are in danger, everyone and their dog, plus the stock markets, panic. Otherwise, not so much. It’s only those poor suckers down there in the south, you know. We’ll see the same when climate change really threatens the industrialised countries.


  1. Note: YouTube and Google inject extensive tracking for embedded video, hence please watch directly on their website. Not that they don’t track you there, mind you, but I don’t want to contribute to their racket from this site. It would also violate my privacy policy. ↩︎

  2. Unless they’re savvy and yank out their phone calculators… in which case it would not an estimate anymore, I guess. ↩︎

  3. Podcast interview with Sam Harris on 11 March. One of the best levelheaded discussions I have heard. It’s only one hour long, and worth your time of you want facts-based information from a scientist and practitioner. ↩︎

  4. Mostly existing drugs, eg. for Ebola and Malaria, being repurposed. ↩︎

  5. On the bright side, these viruses, with all their variability and adaptability, show evolution at work. Take that, creationists. ↩︎