The same day, the United States also confirmed its first case of Covid-19.

But what happened next in both countries was really different.

This chart shows, for several countries, the number of Covid-19 tests performed

relative to the population.

Right away, South Korea started aggressively testing for the virus.

Before long, it had conducted more tests per 1,000 people than any other country.

Most experts say that testing at that scale is the key to safely opening public places.

“We need to test and trace..”

“We need more testing.”

“The larger the outbreak you have of the virus, the more tests you should be doing..”

It's why South Korea was able to avoid a broad lockdown.

The US was much slower to ramp up their testing.

But eventually, in mid-April, they caught up to South Korea.

As of mid-May, about 3% of the US population had been tested for Covid-19.

And now, some parts of the US are starting to reopen.

But some experts warn that in many of those places, reopening is premature;

that even though the US has been doing a lot more testing,

they haven't been testing the right people.

Because South Korea ramped up testing so quickly in those first few weeks,

they were able to track down lots of people who have the virus, and isolate them,

so that everyone else could go back to work.

And most experts agree that if you send people back to work, without doing this first,

it's really dangerous.

“Testing is like response 101.

It's the most obvious thing we need to do in any disease outbreak."

That's because, if a country doesn't test effectively,

they don't really know where the virus is and how many people have it.

So: how do you know if you're testing enough to avoid this?

This chart shows, for each of these countries, what percentage of all of their Covid-19 tests

have come back positive, up until mid-May. It's called the test positivity rate.

This number doesn't tell us what percentage of people in each country have Covid-19.

But you can think of it as an easy way to see which countries are doing enough testing

to contain their outbreak.

“We would certainly like to see countries testing at the level of ten negative tests

to one positive.”

That's 10%.

In other words, the countries below this line have done a better job testing.

And that's because this number can tell us a lot about who in a country is actually

getting tested, and how.

“What we know about this disease is, there are a lot of people who have very mild symptoms,

who have actually no symptoms at all, and we're almost surely not testing those people."

In the US, Covid-19 tests were in short supply for a long time.

And that shaped the Center for Disease Control's guidelines

on how health care workers should allocate testing:

Prioritize people showing up to hospitals, with the most severe symptoms.

If that's where testing is focused,

it makes sense that a large portion of people tested will test positive.

But it also means you're missing anyone who isn't actively seeking out those tests.

This is a reactive testing scenario.

It's probably what happened in other countries with high test positivity rates.

And to understand why that matters, we can look at countries

that also experienced a big outbreak, but have a lower test positivity rate.

Like Australia, Germany, and South Korea.

Instead of narrowly focusing on testing the very sick, these countries tested proactively.

They tested a much wider group of people, including many who didn't feel sick at all.

“They're testing lots and lots of people and they're probably picking up everybody.”

That led to a lot of negative results, which drove their positive rate lower.

And because they tested so many people, their tests didn't only find people with severe symptoms,

but also those with mild symptoms,

and even many who tested positive, but had no symptoms at all.

And these are the people who are key to containing an outbreak.

If someone doesn't feel very sick, it's unlikely they will seek out a test.

Which means you have to find them.

And when testing is only reserved for the sickest,

these people, who do feel sick, often don't make the cut.

“They said, you have a light fever, but on the lighter end.

So I didn't get tested because of that.”

“I developed a dry cough. And I called my doctor.

I'm also pregnant, so I called my prenatal doctor as well.

And they both said no to getting the test; that they didn't have access to it,

and that I didn't qualify.”

“Right off the bat, they told me, OK, we're going to assume you have it,

and we're not going to test you."

If you chart the United States's test positivity rate over time, you can see it's been going down,

which could be a sign the US is starting to test more effectively.

But if you break it out by state, you can see that some places still have a long way to go.

In a reactive testing scenario, the severely symptomatic people may have gotten tested,

but the mildly symptomatic, and the asymptomatic

are mingling with people who don't have covid-19.

To find those people, and lower the test-positivity rate, there are 2 options.

One approach is to cast the net wide, and randomly test as many people as possible.

"If you just blanket the country with tests. Anybody who wakes up every morning,

every other day, if you want to go get a test, you can get a test.

Every time you go to work, you can get a test.

Asymptomatic people are getting picked up,

just by the fact that everybody is getting tested all the time."

This would require a huge increase in testing.

In the first week of May, the US was averaging about 260,000 tests a day.

But a random testing approach could require tens of millions of daily tests.

The other approach requires less testing, but is more targeted,

starting with the people we already know tested positive for covid-19.

"You isolate them. But you also talk to them about all their contacts. Everybody they have

spent any time with. And then you go test all those people. And some of them are going

to be asymptomatic and you're going to pick up asymptomatic disease through contact tracing."

Experts like Dr. Jha say that approach would require more like 900,000 daily tests.

But in both scenarios, the goal is the same:

"The entire math of this comes down to one simple idea.

You want to keep infected people away from susceptible people. That's it."

Safely transitioning out of social distancing

requires finding out where the virus is, and who has it.

That means testing.

And it's why, if you want to know if your country, or state, or city,

is doing a good job containing its outbreak, its test positivity rate is a good place to look.

Because the difference between opening back up with a low test positivity rate,

and a high one,

is the difference between caution,

and just hoping for the best.