A Mathematician’s Guide to How Contagion Spreads - WIRED

His new book, The Rules of Contagion: Why Things Spread—and Why They Stop, lays out those tools and how they can be applied to other parts of life.

Now we’re here rediscovering the effectiveness of things like masks.

AK: I think that's a very important point.

We know what measures work, so why aren't people doing them.

Why aren't people just doing what they should be doing to reduce transmission?’ And of course, we've now got major epidemics in a number of countries that supposedly had high levels of preparedness.

But transmission is not going down, because people aren't following those supposedly obvious things that can help reduce transmission.

It's not the case that one group or one country is going to have the answers?

Essentially, you establish the contact-tracing network and then vaccinate people rather than just quarantining them.

But again, that's very reliant on trust, because you need people to give that information to contact tracers.

You need people, when they're symptomatic, to come forward.

And of course, if you've got diseases that are associated with stigma, then that can reduce the kind of engagement that people have with health systems.

And again, it's the same for Covid—that if we want to use these targeted measures like contact tracing, we need people to report when they're symptomatic.

We need them to contact people who might be exposed as well or engage with contact-tracing systems.

So it's those same basic principles of really taking it seriously as a threat and engaging with the measures that we know can help reduce transmission.

It's just a different setting in a slightly different period of time.

We can target any of those four things to reduce transmission.

AK: One of the really striking things about this infection is that it's sort of a middle case in terms of our ability to contain it.

So it's not like SARS, which has features that mean contact tracing can bring it under control in many countries.

But it's also not like the flu, where it just spreads so rapidly and so few people have symptoms that there's very little hope of controlling it.

It's totally obvious that's what's going on, and people aren't really drinking dead mosquitoes.

But people just hadn't made that step yet.

So I think, first of all, it's important to realize that some of the insights that are obvious are actually only possible in hindsight.

But I think it's also accepting that you may not have perfect evidence, but if you have sufficient evidence that there's a threat, you still have to act upon it.

And there's obviously been a lot of debate, with many groups speculating that perhaps it's much lower and perhaps many more people have been exposed.

You have to piece together from observation what you think might be going on and ascertain the level of threat you're dealing with.

Ultimately, I think that's where epidemiological thinking can be very useful.

And then, of course, other forms of offline contagion, like violence.

It sounds like people had thought a lot about financial networks in terms of how you could profit from them, but not so much in terms of how they could fail.

AK: I don't think, at that point, there was much thinking about how things were linked together.

I think that's when people really began to understand the role of network effects, particularly some of these features where you had these large banks widely connected to create these hub effects, which meant that contagion spread far wider than it might have otherwise.

So one is obviously things like capital requirements, particularly on banks that are structurally important to the network.

But talking to people who work for central banks, as I wrote in the book, I think it is very clear that once you actually start changing the structure of the network, then that becomes much harder to persuade people to do.

So I think you can end up with a situation, unfortunately, as we often get with infectious diseases, where you can understand how contagion is happening, but actually implementing things that reduce those risky links may come in for a lot of opposition.

Your conclusion was that misinformation isn't as infectious as people might think, at least compared to very infectious diseases like measles.

So actually trying to trace down bits of content and reactively remove things or stop people from sharing them is going to be very difficult on that kind of timescale.

A lot of things that are posted aren't shared by anyone.

For example, WhatsApp in recent years has changed how much things can be shared with large numbers of people to reduce the spread of misinformation.

So I think it's those structural changes, or those preemptive attempts to reduce susceptibility to this kind of messaging, which probably can be more effective than trying to frantically chase down an outbreak that's occurring on a timescale of seconds.

And of course, if you can't see any one of those steps, then it will look like the outbreaks just come from nowhere.

So I think we need to understand more about this ecosystem of things going across platforms, being amplified in different directions.

I think that reflects how we'd go about targeting a contagion, whether it's a biological one, whether it's malware, whether it's financial.

So it really suggests that if you look at the situation and it seems like a few mild infections in younger groups, that may well be the point to to think about what's going to come further down the line, rather than waiting for a situation where you’re almost overwhelmed.

AK: Unfortunately, I think it's a similar situation in many areas of Europe and other countries.

I think, as a society, we’re going to have to navigate the privacy trade-offs and maybe accept that, if we want to bring this under control, we need certain sorts of data?

Certainly also countries like Thailand and New Zealand have floated getting people to check into venues with QR codes and that sort of thing?

But I think it is a balance that we're going to have to deal with, because if the alternative is we have widespread lockdowns, then that's very disruptive to a large city in a different way.