Counties with the highest and lowest COVID-19 test positivity rates

For the first time since America's COVID-19 outbreak started, a federal public health agency released county-level testing data. On Sept. 3, the Centers for Medicare & Medicaid Services (CMS) published a dataset showing COVID-19 test positivity rates broken out by U.S. county. While this dataset has not been highly publicized, it provides key information about the state of COVID-19 testing and outbreak control across the country.

A test positivity rate describes the percentage of tests conducted in a particular area that return a positive result. For example, if 1,000 people in a New York county are tested for COVID-19 in a particular week and 10 of those people test positive, the county would have a positivity rate of 1%. Meanwhile, if 25 people in an Oklahoma county are tested and 5 test positive, the test positivity rate for that county is 20%.

These positivity rates are typically reported for a short period of time, either one day or one week, and are used to reflect a region's testing capacity over time. If a region has a higher positivity rate, that likely means either many people there have COVID-19, the region does not have enough testing available to accurately measure its outbreak, or both. If a region has a lower positivity rate, on the other hand, that likely means a large share of the population has access to testing, and the region is diagnosing a more accurate share of its infected residents.

Test positivity rates are often used as a key indicator of how well a particular region is controlling its COVID-19 outbreak. The World Health Organization (WHO) recommends a test positivity rate of 5% or lower. This figure, and a more lenient benchmark of 10%, have been adopted by school districts looking to reopen and states looking to restrict out-of-state visitors as a key threshold that must be met.

Stacker has visualized the positivity rates reported by the CMS. These figures reflect average weekly test positivity for the week of Aug. 27 to Sept. 2. CMS has categorized counties according to their positivity rates:

• Green: test positivity under 5%(or with fewer than 10 tests in the past seven days
• Yellow: test positivity between 5% and 10%
• Red: test positivity greater than 10%

The visualizations also include state-level positivity rates for the same time period, sourced from the COVID Tracking Project at The Atlantic, and county-level case and death counts from the most recent week of data available (Sept. 3 to Sept. 9), sourced from the New York Times COVID-19 data repository.