America’s 20 percent positivity rate is disquieting. The U.S. did almost 25 times as many tests on April 15 as on March 15, yet both the daily positive rate and the overall positive rate went up in that month. [...] when tests are rationed so strictly, only people with severe symptoms make it into the testing pool, ensuring that the positivity rate will be extremely high. [...]
The test-positivity rate, then, is a decent (if unusual) proxy for the severity of an outbreak in an area. And it shows clearly that the U.S. still lags far behind other countries in the course of fighting its outbreak. South Korea—which discovered its first coronavirus case on the same day as the U.S.—has tested more than half a million people, or about 1 percent of its population, and discovered about 10,500 cases. The U.S. has now tested 3.2 million people, which is also about 1 percent of its population, but it has found more than 630,000 cases. So while the U.S. has a 20 percent positivity rate, South Korea’s is only about 2 percent—a full order of magnitude smaller.
South Korea is not alone in bringing its positivity rate down: America’s figure dwarfs that of almost every other developed country. Canada, Germany and Denmark have positivity rates from 6 to 8 percent. Australia and New Zealand have 2 percent positivity rates. Even Italy—which faced one of the world’s most ravaging outbreaks—has a 15 percent rate. [...] New York City’s positivity rate is an astonishing 55 percent.
米国当局はこの問題と、陽性率を10％未満に下げるWHOガイドラインを実際に認めています。April 22 news：
But how much testing is enough?
There's no exact number to aim for, but here's a guiding principle: You want a low percentage of your tests to come back positive, around 10% or even lower, says William Hanage, an epidemiologist at Harvard.
That 10% benchmark is based on recommendations from the World Health Organization. Why should positives be low? If a high percentage of tests come back positive, it's clear there's not enough testing to capture all of the infected people in the community. "The lower the percentage of tests you're doing that come back positive, the better," Hanage says.
Some countries that have done extensive testing have positive rates near this 10% benchmark, or lower. South Korea is "testing so many people that only 3% of them are positive," said Rochelle Walensky, an infectious disease specialist at Massachusetts General Hospital, during a livestream hosted by the medical journal JAMA.
But, so far the U.S. appears to fall short of this benchmark. Nationally, according to CDC data, about 18% of tests have been positive to date, and 21% were positive in the week ending April 11. [...]
Admiral Brett Giroir, a member of President Trump's coronavirus task force, acknowledged the importance of this benchmark. "This is a good metric that you want to get about one positive for every 10 tests," he said during the White House coronavirus task force briefing Monday night [Apr 20?].