Contact Tracing: One Path Out Of Social Distancing

Published in

The Startup

8 min readApr 13, 2020

There’s been a lot of news recently about “Contact Tracing”, with a statement from the director of the CDC that aggressive contact tracing is needed, and a joint announcement from Apple and Google that they’d open up better support for contact tracing in their mobile devices.

In this article, I’ll briefly cover what contact tracing is, how it’s related to social distancing, and why it’ll be such a critical tool in the coming months.

Contact Tracing In Context

In an epidemic, if you don’t have a vaccine, there are two broad categories of ways you can slow down transmission: individual (like contact tracing) and collective (like social distancing).

At the individual level, the goal is to quarantine infected individuals quickly enough to prevent further spread. This depends, of course, on knowing who’s infected.

Obviously, the most reliable way to determine who’s infected is through testing. But that’s not as easy as it sounds; for one thing, it’s hard to actually get millions of tests created quickly for a new virus like COVID-19, so we simply don’t have enough tests (as everyone has seen). For another thing, testing doesn’t actually catch everyone who’s infected; people who are in the early stages of infection aren’t yet shedding enough virus to test positive. If you get a “negative” test at that point, that’s very dangerous–you might end up spreading the virus even more, because you think you’re safe.

So in addition to testing, we also use another individual method called contact tracing. Here’s how it works: for every infected person, you figure out who else they’ve come in contact with, and then potentially who those people have come in contact with, and so on, building lots of small graphs of potentially infected individuals (who can then quarantine even if they’re not feeling sick yet). When contact tracing is fast and thorough enough, only those individuals who are potentially infected have to self-quarantine, leaving the majority of people, as well as the economy, mostly unscathed.

(If that sounds a little confusing, there’s a nice graphical overview from Nicky Case, with epidemiologist Marcel Salathé, here.)

Contact tracing is a really useful tool; however, as it’s practiced today, it is very manual. You ask someone to remember where they’ve been and who they’ve seen, and then you call up each of those people in turn and interview them in the same way. This is extremely slow work, taking days or weeks to complete. And it can only be done by trained professional health care workers, which means it’s difficult and expensive to scale. When you’re up against a virus (like COVID-19) that spreads rapidly through air or surfaces, and where some infected people don’t even show symptoms at all, manual contact tracing is unlikely to stop the spread in time.

… which brings us to where we are now. We’ve roundly failed to contain the COVID-19 outbreak using individual techniques (manual contact tracing or widespread testing), so our only recourse has been a move to collective techniques: namely, mass social distancing. We tell as many people as possible to isolate themselves at home and cease all social contact outside of their own families, regardless of who they’ve been in contact with. The impact of this step is obviously massive (as we’re all discovering), and we’ll be recovering from it for years. But, the alternative–millions of people dying–is worse. So social distancing is our only option right now.

The Social Distancing Roller Coaster

What I’ve described so far might give you a mental model like this: we can pursue contact tracing up to a point, but after that, social distancing becomes necessary:

This is true, but it’s only part of the story.

Imagine that in a few weeks, we see the rate of new infections drop, and we move to relax social distancing measures. Businesses begin to reopen, and everyone breathes a sigh of relief.

And then … the infection rate spikes again, sending everyone back into quarantine. What happened? The key number here is called the Basic Reproduction Number (usually abbreviated as R₀), which is the exponent with which the epidemic spreads every day. If R₀ is greater than 1, the epidemic grows quickly. If R₀ is less than 1, the epidemic goes away on its own. Full social distancing seems to bring us very close to R₀=1, but even a tiny movement up above 1 means things are still getting worse, not better. (For a really interesting look at the mathematics of how epidemics spread, watch this incredible video from Grant Sanderson.)

So if we relax social distancing and that moves the R₀ value up above 1, the virus will just start growing again, until it forces us back into social distancing. So, we’re looking at waves of infection and social distancing like this, potentially for years. This isn’t far-fetched; on the contrary, many experts see it as a likely scenario.

This could potentially be more destabilizing than the initial onset of the pandemic, because it takes away people’s hope that this might be a short-term problem.

Contact Tracing To The Rescue

But this isn’t inevitable, because on the other side of the peak, contact tracing actually becomes a viable option again:

In the period after the peak, society will be in a balancing act, trying to open things up as much as possible while still keeping R₀ below 1. This means either using expensive collective techniques (yay, more social distancing!), or finding better ways to do individual techniques (mass testing, contact tracing, and vaccinations).

We can hope for more testing or a vaccine, but they’re not guaranteed. The only sure thing is contact tracing. With concerted effort, we could actually get out in front of new cases once we’re past the peak, and rapidly trace the infection. With enough participation, it’s conceivable that we could prevent (or at least localize) further spikes.

The trouble, of course, is that traditional contact tracing is painfully slow and requires thousands more qualified health care workers than we have available. Would it even allow us to respond to new infection spikes, and prevent a full relapse into social distancing? Probably not. We can (and should) do as much manual contact tracing as we can, but even our best efforts will have big scalability problems.

But, who says we just have to do traditional contact tracing?

There’s An App For That

This isn’t the world’s first pandemic. But it is the first pandemic where a majority of the people in the world carry a networked, location-aware supercomputer in their pockets.

So now imagine a different vision of how the next few months might play out. As we lift social distancing measures, people begin to return to work and social interactions. But, they carry an app on their phones that automatically collects all the data needed for contact tracing. At that point, if you feel sick, you don’t need to remember who you’ve been in close proximity with: your device knows exactly who you’ve been in close proximity with. And more importantly, it can rapidly share that information with those people, who can perform the same calculations on their recent contacts, and so on. The process that used to take weeks can now happen in minutes, which means that there’s a much higher chance of actually stopping further transmission (rather than just tracking it).

And more importantly, this can work even on anonymous interactions, like sitting near someone in a coffee shop. With manual contact tracing, all you can say is that you went to a public place and spent time there; you can’t (even in theory) call all the other people who were there, because you don’t know who they are. But with an app, those connections can still be built. If the app tells me that someone sitting near me at the coffee shop three days ago turned out to be infected, it doesn’t matter if we know each other; I can go get priority testing, or maybe even quarantine myself for a period of time to protect others.

Contract tracing can make a huge difference. This isn’t just my opinion; a study from Oxford University suggests that instantaneous contact tracing like this could actually help reduce the R₀ to less than 1.

This is the vision behind the announcement from Google and Apple–creating a framework that allows developers to build digital contact tracing apps like this, which could allow us to open up social distancing measures without dipping back into accelerating spread. Digital contact tracing isn’t perfectly effective, of course; the aspiration isn’t to catch every single case, but simply to keep the R₀ low enough.

To put it in context of other ways we could attempt to come out of this pandemic, watch this short video by Nicky Case. (Spoiler alert: contact tracing is the only one that ends well.)

One Does Not Simply Do Digital Contact Tracing

Now, obviously, there are also serious concerns with digital contact tracing, particularly around privacy; is all this location data being sent to a central database? Can this data be used for other less noble purposes, or stolen by hackers? Are sick individuals putting themselves or their families at risk by sharing their status? Do people with demonstrated immunity get special treatment?

These are just a few of the important questions to ask. For a thorough overview of the risks, there’s a great paper from the privacy group at the MIT Media Lab:

Apps Gone Rogue: Maintaining Personal Privacy in an Epidemic

I cover the importance of privacy in a separate article, here, because it really requires a deep dive. But for the moment, just note that it’s possible to enable digital contact tracing without giving up your privacy, by combining public data about the epidemic with private data kept only on people’s trusted personal devices.

If you look at the white paper from the joint effort between Google and Apple on their BLE tracking, it claims that the techniques in question do not collect personally identifiable information or user location data, and don’t share your contact lists outside your phone. Obviously, we’ll need to see a lot more details about this as it develops, but based on what we’ve seen so far, it seems like these apps can be privacy-preserving while still being effective at slowing the spread.

It’s important to understand this distinction, precisely because this is such a huge opportunity for us. The key message is: we can restart the economy more quickly and reliably if digital contact tracing is one of the tools at our disposal, and that doesn’t have to compromise your privacy.

I’m Ian Varley, a software architect living in Austin, TX. I am not a trained epidemiologist, nor do I play one on TV; I’m just a concerned layperson trying to help people sort through a mass of information. The statements in this article are entirely my opinion, not the opinion of my employer. As much as possible, I’ve tried to include only statements that are backed up by other sources, including the inline linked articles, as well as the following references:

Thanks also to Tom Dubois (an actual epidemiologist!) for additional feedback.

If you see any misleading or inaccurate information , please comment!