Why do we still need testing?

By Ashish K. Jha, Faculty Director, HGHI

Over the past few days, many federal policymakers have been arguing that testing is overblown as an issue – that it is not so important or that we are doing enough. These are two separate issues. Is testing really important? And if yes, are we doing enough?

In this blog post, I’ll try to lay out in simple terms why testing matters. In the following piece, I’ll walk through the work our group has been doing to estimate how much testing we should be doing – the logic behind that estimate.

So, let’s get started. Why do we need testing?

Why is it important to have “enough” (whatever that might mean – see the this blog post for an explanation)? It’s worth taking a quick stroll through the basic biology and epidemiology of an outbreak.

The tale of Amit and Frank

Say there are two guys – Amit and Frank. They are co-workers and friends but live separately.

Amit has contracted the virus, SARS-CoV-2 , and came in contact with Frank, who doesn’t have the disease and isn’t immune (because he’s never had the disease). There is a chance Amit will spread it to Frank. Based on what we know, on average, Amit will spread it to 3 susceptible people. Those 3 will, on average, spread it to 9 and those to 27 and those to 81 and 243 and so on. Exponential growth. It’s bad.

How do we stop it? Barring a vaccine, we can’t easily. We want to keep Amit away from Frank but if they are hanging out, Frank has a reasonable chance of getting it.

The key is to prevent Amit from hanging out with Frank. That’s the whole point of shelter in place: to dramatically reduce the chances that Amit comes into contact with Frank and, therefore, Frank’s chances of getting infected.

But what happens after we open up again? Amit will come to work and see Frank. They might have coffee together. And voila, Frank might get infected. More broadly across society, as we open up, infections will take off again. Exponentially. It’s biology.

Robust testing means we don’t all have to stay home

So, what do we do?

We can stay shut down. Forever – or at least until we have a vaccine. But that’s terrible for the reasons we all know.

How about if we just made Amit shelter in place (he’s infected) and let Frank and everyone else not infected go about their lives? Isn’t that what we want? Yes.

How do we do that? We test lots and lots of people – find all the Amits of the world and get them to shelter in place, making it clear to the Franks of the world that it’s safe for them to go to work or school or a restaurant because Amit is at home.

If you can’t test, then you can’t identify the Amits of the world, and the only way to protect Frank and prevent exponential growth is to force Amit and Frank and everyone else to stay at home.

So fundamentally, we want to be doing so much testing that we can capture most of the Amits of the world, knowing a few will slip through, but we will pick up enough of them to stop the disease from exploding.

If you can’t do enough testing to identify most of the Amits of the world, there will be a lot of Amits out there continuing to spread the virus, and exponential growth means that cases will continue to rise. The untested Amits will cause the outbreak to get out of hand, and we will have to shut down again. It’s biology – and math.

If you want to have an open economy, you have to want ubiquitous, plentiful testing. Testing, tracing, and isolation are the pre-requisites for liberating the economy. Anyone who tells you that we don’t need large amounts of testing isn’t serious about controlling the disease, or serious about keeping the economy open.