Understanding the data will help us understand the danger

In Australia right now, both Sydney and Melbourne are battling outbreaks of the delta variant of covid, and wondering why it just doesn’t seem to lie down and die the way the virus did in Australia last year. There are many reasons for our current problems – too, too many of them political – but a lot of it comes down to the R0 value, or how contagious this variant is.

Last year’s variant was not nearly as contagious – some estimates say delta is three times as contagious as that one – and so it was easier to put down. But what is R0, and why do we care?

R0 is known as the “basic reproduction number”, and simply represents how many people an infected person will infect. So an R0 of 2 means that each person with covid spreads it to 2 other people. That means the infection doubles at every step, and that’s bad. 1 person infects 2 others, they spread it to 4, who spread it to 8, etc. At the 6th step, we’re already at 64 new people being infected. Left unchecked, at the 10th step we’re at 1024 new people infected.


Delta has an estimated R0 of 5. Every person with delta will likely spread it to 5 other people. That means 1 case becomes 5, those 5 infect 25 more, those 25 infect 125 more, and it escalates alarmingly fast. Left unchecked, at the 10th step we’re at nearly 2 MILLION new people infected.


If we graph the first 6 steps, we can see how an R0 of 2 compares with an R0 of 5.

New infections for R0 of 2 versus R0 of 5. R0 of 5 is a red line jumping sharply upwards, to over 3000 by step 6. The blue line for R0 of 2 barely registers on the graph, it is relatively low and flat.

Or, to put it another way, check out these little coronaviruses. Each step is 5 times the step before.

5 steps of transmission of the delta virus, with an R0 of 5. Each little covid image represents one new person infected

The good news is that we can effectively modify the R0. The delta variant’s R0 is only 5 if people are interacting in a way that allows the virus to jump from person to person. This is why contact tracing can be so effective – if we can identify all of the people who have had contact with an infected person BEFORE they become infectious themselves, then we can prevent the virus from spreading any further. If we can drop the R0 to under 1, meaning that, on average, people infected with delta are infecting less than one other person, then we can get it under control.

Contact tracing works if we can identify every single person an infected person has been near enough to to transfer the virus. The trouble is that delta makes that jump so quickly and easily that contact tracing needs to identify everyone an infected person has walked past in the street, especially if they were unmasked. This is obviously not achievable. Plus, even the most dedicated of us sometimes forget to check in, so that is not 100% reliable either.

Wearing masks helps, but only if they are properly fitted, worn properly, and washed or replaced regularly.

This is why we need strict lockdowns. Because lockdowns reduce the number of people we each come into contact with, thus effectively reducing the R0, and limiting delta’s opportunities to spread.

If we don’t get the R0 down, we can’t control the spread of the virus. It’s as simple as that.

And if you’re wondering why I stopped my little covids at step 5 in the picture above, it’s because step 6 is too big to fit on the screen.

Step 6 is 3125 new cases – too many to display onscreen (this is not the full set)

Once again, our safety relies on our leaders (and the public) understanding exponential growth. It’s not actually complicated, but few of us have ever needed to know how it works. Now, more than ever, we need to build our data literacy, so that we can understand the danger we are in.

For more reasons why we need to be data literate, and how to teach our kids to change the world, check out Raising Heretics, pre-order now, or buy online in all the usual places from August 1st.