The 20860 ZIP code, near Olney, claims the highest per-capita rate of confirmed coronavirus cases in Maryland according to an analysis of state data by Route 1 Reporter. As of April 19, 2020, 27 residents of the 20860 ZIP Code, population estimated around 2,400, had a per-capita rate of 1,127 cases per 100,000 residents, far ahead of the next most-heavily-affected ZIP code, Colmar Manor’s 20722, which has 5,710 residents, 33 confirmed cases for a per-capita rate of 578 cases per 100,000 residents. These insights are the result of new ZIP code-based case counts from the Maryland Department of Health mashed with population data from the U.S. Census.
Last week, Maryland began to release coronavirus case information by ZIP Code. But ZIP Codes are really only good at one thing: coordinating the distribution of mail. ZIP codes vary widely in both their physical size and the size of their populations, making it hard for one to make accurate at-a-glance comparisons between ZIP codes about the scale of the pandemic from raw case counts alone.
One way around this is to calculate the number of cases on a per-capita basis. In the world of health, crime and social statistics, the most commonly used is the rate of disease or injury per 100,000 residents. You get this number by dividing the overall population of a given territory by the number of documented coronavirus cases and multiplying the result by 100,000. By normalizing the data in this way, one can make rough comparisons about infection rates across communities of differing sizes.
The map below takes Maryland’s publicly-available ZIP code coronavirus case counts, mashes them together with population data from the U.S. Census Bureau to produce a rough per-capita rate of coronavirus infection for every ZIP code in Maryland.
Now, the caveats. You need to be aware that Maryland is not releasing case numbers for ZIP codes with fewer than seven confirmed cases. Also, the population data dates to the 2010 Census, because that’s the last time the Census Bureau updated its ZIP code population tables. The population numbers are certainly out of date by this point, but this exercise assumes the disparities in the underlying populations between each ZIP code has remained more or less constant over the past decade.
More specific to this situation, you also need to heavily weigh how testing availability and the range of access to health care across Maryland is suppressing the case numbers we are presented with. As Prince George’s County health officials have emphasized this week, the case counts don’t paint the full picture, but they give a hint at its shape.
Our second map, below, takes that same data to show how confirmed case counts have changed between April 13 and April 16, 2020. Here, the darker colors show areas where case counts have more than doubled over the past six days.
Editor’s note:Route 1 Reporter is – normally – a subscriber-supported local news website. Due to the state of emergency, articles about the coronavirus pandemic will be available for free. If you like the reporting, please support Route 1 Reporter on Patreon.