How it works
Imagine a city — we’ll call it Metropolis — that’s dealing with the COVID-19 pandemic.
Two weeks ago, this imaginary city did about 100 p ² came back positive. This means that the city ² per day, and that the positivity rate was 5%. Metropolis has not counted the true number of coronavirus cases in its city, however. Many people who have COVID-19 — and no one knows how many — are not being counted by medical authorities. Because COVID-19 can have mild symptoms or even none at all, many people with the disease are unaware they have it. Others may have symptoms, but be unable to access testing near them.
This means that if testing increases, the number of positive cases reported is also likely to increase.
Last week, let’s say Metropolis doubled the number of tests to about 200 per day. If the number of true cases had stayed about the same, we might expect the number of positive tests to double, as well. This means 10 tests would be positive, which would still mean a 5% positivity rate.
But what if the week they tested 200 people, they found 20 positive cases? The positivity rate would have doubled, from 5% to 10%. This would mean that the number of positive tests is increasing at a faster pace than the number of tests overall.
When the positivity rate increases alongside the number of reported cases, it means that the number of cases is growing, but that it’s likely not because of increased testing. The same goes for decreases in positivity rate alongside decreases in reported cases.