COVID-19: The Rule of 72 Rules

By Shlomo Maital

Albert Einstein once said famously, that compound interest is the “most powerful force in the universe.”   It is indeed, especially when compound interest is at work, in transmitting virus from one person to another, at varying rates.

So, here is the Rule of 72, and some optimistic news from my country Israel.

How fast is the number of infected persons doubling? How often? Are we succeeding in slowing it? The graph above shows the number seriously ill, or who have died, from COVID-19, in Israel,  dated from when schools closed – and the dotted line shows the future projection. The data are by the National Security Council.

Initially, it was assumed that the daily growth in COVID-19 seriously ill and deaths was growing at a compounded interest rate of 25% daily !! (why compound: because 25% more people, infect 25% more people, infect 25% more people..and so on…like bank compound interest).

Here is the Rule of 72:    The Rule of 72 shows you how quickly you’ll double your money. All you have to do is divide 72 by the interest rate it’s earning. This is the number of years it will take for your money to double.

Translate that to virus: Divide 72 by the daily rate of increase at which people fall seriously ill or die – that tells how rapidly the number of victims doubles – in how many days. 25% daily growth? 72/25 = about 3. Every 3 days. So in one month, 30 days, we have 10 doublings.   2 raised to the 10th power is 1,024. A thousand times more ill and dead, in one month. Simply not a situation hospitals can handle.

And that was the initial doubling rate in Israel, in the early days of school closure. Then people got the message and sheltered at home.

The daily infection rate then declined, to 15%. According to the National Security experts.

What does that mean? Rule of 72:   72/15 or about 5.   Doubling every five days. How many doublings in 30 days?   30/5 equals 6.   Two to the 6th power equals 64.

Sixty-four times more victims.

Something we can handle. Difficult, tragic – but – do-able.

   Sixty-four times more victims is hugely better than 1,024 times more victims.

   This is the Rule of 72. And the key point is – the Rule of 72 is in OUR hands. As my cell phone tells me each time I open it: Stay Home.  Getting the ‘compound interest’ rate down from 25% to 15% is hugely valuable.  When we invest we want more interest; when we shelter, we want a whole lot less (virus spread).  And even small differences in the rate of spread make a huge improvement.

   Now – next step, how do we figure out who can emerge from home to run the factories and farms? And when?   But so far, the Rule of 72 Rules.