The graph from the right is the same one shown above. The three other graphs show what happens if there is a delay of one day, two days, or three days in isolating symptomatic and asymptomatic people.

Let’s look at the graph all the way to the left. It’s basically telling us: “If you have a 3-day delay in isolating and quarantining cases, it will be awfully hard to stop the disease.” Every bit contributes, but with this type of delay, it contributes very little.

The second graph tells us: “If you have a delay of just two days in both isolating and quarantining, you need to be able to isolate at least 70%-90% of infected, and trace at least 70%-90% of their contacts to stop the epidemic just with this measure.”

Put another way: you can control the epidemic without shutting down the economy and just with this single group of measures if you’re fast and effective at testing people, isolating the sick, tracking their contacts, and quarantining them. You need to do that super fast and really effectively, or else you won’t be able to control the epidemic just with this measure.

Without executing this set of measures well, it will be awfully hard to control the epidemic, and you’ll be forced to either find some other miraculous set of measures, go for herd immunity, or apply another Hammer — with all their economic costs or massive deaths.

This is why it’s so important for countries to have lots of testing that works as fast as possible. You need both: the quantity and the speed.

South Korea’s drive-through testing and phone booth testing offer a model. The easier it is for people to get tested, the more people will do it quickly, and the better we will control the epidemic.

Some countries are considering testing everybody all the time. Imagine, for example, that most of the population of the US was tested every week — say 300 million of the 330 million. That would tell the country everybody who is getting sick at any time, and likely control their epidemic. Doing 300 million tests every week is a bit far off now, however, and might be a bit expensive. In a year, that’s over 15 billion tests. If we assume they’re very cheap because of the volume — let’s say $20 each — that’s $300 billion, which is quite expensive, even if just 15% of the $2 trillion stimulus.

This is extremely expensive and not realistic today. But if we could have efficient ways to test more people, the calculation could change.

There are ways. For example, this paper explains how, if few people are infected (i.e., if your prevalence is low), you can cleverly test a bunch of people at once and reduce the number of tests needed eightfold. Dropping the cost of mass testing from $300 billion to less than $40 billion would be a huge improvement. Many countries are already doing it, such as Germany, Austria, Israel or the US.

Another very promising approach is sewage testing.

The idea is that we can measure how much coronavirus there is in sewage, which can tell us broadly how many people are infected. From there, we can test sewage upstream to find the buildings where the virus comes from, test everybody in that building, and isolate the infected.

In summary:

You need to test a lot, to identify infected people as soon as possible.

That means enough testing so that only 3% of your people tested turn out positive, since that’s what successful countries are seeing.

You also need to test very fast, so you can isolate the infected immediately and reduce how many other people they infect.

That is half of the battle. The other half is testing all contacts to identify who else is infected but has not developed symptoms yet. 45% of infections come from them.

Similarly, you want to do that contact tracing very fast, to reduce the period of pre-symptomatic infectiousness.

There are ways to do testing efficiently, testing several people with one single test.

There are other promising approaches to testing, such as sewage testing.

In a perfect world, we can test everybody all the time. We might get there, but in the meantime, it looks expensive and hard.

So in the meantime you need to prioritize who you test. First, people with symptoms. Then, all their contacts.

That leads us to contact tracing.

Note: We will cover serological tests and other testing details another time.

This section draws heavily on ideas and sources from @Genevieve Gee’s research on Testing. Notably, the idea of tracking % of positives is hers.

Contact Tracing

This is by far the meatiest section in the entire article. That’s intentional: The stakes are extremely high. As we just saw, not only can good contact tracing slash transmissions; it’s vital to moving from the Hammer to the Dance, to reopen an economy safely. But it’s also very complex and poses many privacy questions.

But before we dive into it, we need to get a good sense of what exactly it means to trace contacts.

Let’s call Bob the person who has been infected. We want to identify as many of his contacts as possible, as fast as possible. The ones that matter are not all people he’s met, but rather the ones that might have been infected.

To do that, you need a team of contact tracers.

Nurses from the Anchorage Health Department and Anchorage School District work on the COVID-19 contact investigations and monitoring team on Thursday, April 16, 2020 in a conference room at the health department in downtown Anchorage. Photo: Loren Holmes / ADN, via Anchorage Daily News

Contact tracers have several functions. First, they are given a list of people like Bob who have been infected. They interview Bob to learn everywhere he’s gone over the last couple of weeks and who he’s been with. Since Bob is human, he’s frequently unreliable: He might be forgetful, sick, panicking, sad, uncooperative, or all of the above. So contact tracers also use technology to help. An example might be South Korea, where tracers use mobile GPS data, credit card spending data, and CCTV footage. Another example might be using results from a contact tracing app.

With all that information, they put together a list of Bob’s contacts who might have been infected, ordered by likelihood of infection. Then, they call all these contacts. Depending on the likelihood of infection and the government’s rules, they might order them to get tested, self-quarantine, or just check on their symptoms. They want to catch as many contacts as possible, as fast as possible.

But what qualifies as a contact? How many contacts do we need to trace? How fast do we need to trace them?

What Qualifies as a Contact?

Since most people are believed to be infectious for around two weeks only, we only care about people that Bob might have been in contact with over the past couple of weeks. Before that, Bob was unlikely to be infected, and if he was, his contacts are unlikely to be infectious anymore.

Within these two weeks, we only want to identify those likely to be infected. Bob’s family members are all very likely. Conversely, you don’t care as much about the people he crossed on the street 5m away (~15ft).

As we saw in The Basic Dance Steps Everybody Can Follow, contagions are much more likely to happen in confined environments where people are close to one another, speaking, coughing or singing for a long period of time.

Countries convert that into rules. For example, they investigate Bob’s contacts if they spend over 15 minutes together within 2 meters. That sounds reasonable. But in reality, tracers are much more subtle than that. A person that shared a meal for one hour sitting face to face might be marked as high risk and asked to go on quarantine, with investigators checking in every few hours, while a person that shared the line at the grocery store might be asked to just be extra careful and check her symptoms frequently.

How Many Contacts Do We Need to Trace?

We saw before that we wanted to trace at least 60% of contacts and quarantine/isolate them immediately to substantially reduce R (the effective reproduction number, how many infections are caused by a carrier of the coronavirus). But that paper assumed a certain R0 of 2.5 (R0 is the reproduction number in perfect conditions: when nobody is immune yet, and no measures have been taken against it). What if it’s different?

This paper looks at that. It takes different R0s and assesses what share of contacts we need to trace to bring R down below 1. Each line below represents a different R0 of 1.5 (red line), 2.5 (grey line) and 3.5 (brown line).