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  • On January 20th, South Korea confirmed its first case of Covid-19.

  • 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.

On January 20th, South Korea confirmed its first case of Covid-19.

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B1 Vox testing test tested positivity covid

The US tested the wrong people for coronavirus

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    林宜悉 posted on 2020/07/28
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