The principle is that, with the exponential growth phase of an epidemic, individual and institutional actions such as social distancing taken early on can have a much greater impact than if the same actions are taken even a week later.
NJ The magnitude of the outbreak creeps up on you; it doesn’t look like things are growing very much, and then suddenly they are. Today, the U.S. is up to at least 1,714 known cases and we’re only a couple of days on from when it was 1,004. It’s going to be 4,000 by Monday, and then it’s going to be 8,000 by next Wednesday, and then it’s…. Exponential growth is staggering when it takes over.
In a nutshell, what is exponential growth?
BJ Exponential growth is a classic pattern in which numbers stay small initially, but then you end up with very large numbers very quickly. If you start with a certain number, and then multiply that number by a growth factor every day, depending on what that growth rate is, you’ll see the cumulative number doubling over a certain time period.
What really matters is how high that growth rate is. In the U.S. right now, according to Our World in Data, confirmed Covid-19 cases are increasing by about 30 to 40 percent per day and the total number is doubling about every two days.
NJ Think about your family tree and how the number of your ancestors (or descendants) grows with every generation. Or there’s the story about the rabbits breeding. Two rabbits breed four rabbits and four rabbits breed eight rabbits and eight rabbits breed 16 rabbits, and if they’re breeding every six or seven days, very soon you have a lot of rabbits.
BJ If you start with two rabbits and the number doubles every week, you’ve got about 1,000 rabbits after 10 weeks. That doesn’t seem so bad. But after 10 weeks? You’ve got a million. It’s intuitively very hard to grasp how quickly these numbers go up beyond a certain point — people tend to anchor on the low numbers at the beginning, when the curve is relatively flat.
But the same exponential effect is equally powerful with mitigation efforts — staying home now, for example. How do the “exponential now” decisions play out later down the line?