Don's Home Health Coronavirus Pandemic Test Positivity Rate Contact
In "How to Understand COVID-19 Numbers" in ProPublica,
Both Matthew Fox, professor of epidemiology and global health at Boston University, and Youyang Gu, a data scientist best known for his COVID-19 prediction models, advised looking at three measurements together: number of cases, case positivity rates and number of deaths.

Fox says, "Cases going up or down tells you a fair bit about what's going on at the moment in terms of transmission of the virus - but it's only valid if we're testing enough people.

When there aren't enough tests available, as was the case in New York in March, the number of cases reported will be an undercount, perhaps by a lot.
That's where case positivity rates come in: that measures the percentage of total tests conducted that are coming back positive. It helps you get a sense of how much testing is being done overall in a region.

"WHO guidelines say we want that to be below 5%", Fox noted. When a positivity rate is higher, epidemiologists start worrying that means only sicker people have access to tests and a city or region is missing mild or asymptomatic cases."

Positivity Rate August 3
Click on charts to go to the current version

Rates range from 50% in Bolivia to 0.4% in Australia

US Aug 4

Which U.S. States Meet Recommended Positivity Levels? - Johns Hopkins Coronavirus Resource Center

Tests vs Positivity

How to Understand COVID-19 Numbers - ProPublica
Test Positivity: So Valuable, So Easy to Misinterpret | The COVID Tracking Project
Daily State-by-State Testing Trends - Johns Hopkins Coronavirus Resource Center