Analysis: What to make of COVID-19 data?

Analysis: What to make of COVID-19 data?
An illustration of the COVID-19 virus. (Image/Centers for Disease Control and Prevention)

By Denise Dunbar | [email protected]

Data on the Centers for Disease Control and Prevention website indicate that since late October, every state in the continental United States plus the District of Columbia has experienced a sharp spike in COVID-19 cases.

While urban areas have had more total cases, rural regions have suffered more cases per 100,000 of their population. As a percentage of population, the Great Plains region is suffering the worst current outbreak. North Dakota has been by far the hardest hit individual state in the past week with a daily average of 177 cases per 100,000 of population, according to the CDC website.

The data in this chart for both the number of COVID-19 PCR tests in Alexandria and Alexandria’s positivity rate are seven-day moving averages for specific points in time taken from the VDH website on Nov. 20.

By contrast, Virginia experienced the third fewest new COVID-19 cases per 100,000 population during the past week, with a daily average of 23.6, trailing only Maine and Vermont in the continental U.S. In fact, the four-state Southeastern coastal region stretching from Virginia to Georgia was one of three blocks of states, along with the coastal West and upper New England states, with seven-day daily averages of 30 or fewer cases per 100,000 residents, according to the CDC website.

Unfortunately, useful data can be frustratingly difficult to find on federal and state websites and is sometimes poorly or incorrectly labeled when found. 

For instance, the CDC COVID-19 Data Tracker page on Nov. 20 listed in bold type in one of three highlighted boxes at the top, “Cases in last 7 days per 100K: 49.3.” This would lead a reader to conclude that this is 49.3 total cases per 100,000 of population during the past week. Except it’s not; that’s the daily average.

The CDC’s data explanation at the bottom of the page is similarly misleading, as it says: “The map shows total cases per state, new cases in the last 7 days per state, and the rate (cases/100,000) per state.” The word “daily” is missing. The viewer must perform two math calculations to determine that the CDC means daily per 100,000 residents rather than weekly.

It can also be difficult to compare data across different state websites and even within a website, because states track and report their data differently with different time points. Sometimes separate sections of an individual website present data differently.

For instance, Virginia’s COVID-19 website is generally clear and easy to use and has improved considerably since the spring in both the volume of data available and how it’s displayed. And yet, while the fine print explains the differences, it can be confusing to look at the case totals on the Virginia daily dashboard, which include confirmed and probable cases, and then look at the positive test totals on the testing tab, which are only confirmed cases.

The data in this chart is a seven-day moving average of COVID-19 cases in Alexandria for specific points in time taken from the VDH website on Nov. 20.

Of neighboring states, Maryland’s website from the start has been the most comprehensive and easiest to read. Conversely, seven-day average positivity rates are not to be found on either the D.C. or West Virginia websites, though the raw data is present on both sites for readers to perform their own calculations. North Carolina’s site has a yellow line through charts that says “7- day average” – but it doesn’t say what that average is for any given day except the current date.

It’s also important to remember that case numbers are largely a function of how widespread testing is in any state or locality. As more testing is done, a larger number of cases are going to be recorded. Conversely, if a wider net is cast on testing, particularly if asymptomatic people are tested, the positivity rate should go down.

A look at Alexandria’s COVID-19 positivity rate and number of tests reveals the latter point. Back in April and May, before tests were widely available, testing was largely limited to people with symptoms and the positivity rate was high – 32.5% on April 16 and 26% on May 16.

However, once tests became more widely available beginning in late May, the positivity rate began to decline sharply – though this timing also coincided with the ebbing of COVID-19’s first wave in the D.C. region. Alexandria’s positivity rate has remained relatively flat since mid-June, ranging from 6.1% on June 16 to 4% on Oct. 16* back to 6% on Nov. 16, as the average daily number of PCR tests administered in the city has more than doubled during that timeframe.

As more tests are administered, we will get closer to an accurate sense of how many people within each community actually have COVID-19, since the positivity rate merely tells the percentage of those tested who have the disease, not the percentage of overall residents who have it.

For instance, cumulatively since March, 3.1% of Alexandria’s total population has been diagnosed with either a confirmed or probable case of COVID-19, or 4,935 out of 160,530 residents. In the seven days leading up to Nov. 22, a total of 301 residents, which is .19 – or about 1/5 of 1% – were diagnosed with either a confirmed or probable case, according to data on the Virginia Department of Health and the U.S. Census Bureau websites.

These numbers don’t account for people who were asymptomatic but infected and weren’t tested, or for residents with symptoms who haven’t sought testing or medical care. This means the actual percentage of residents who either have had or currently have COVID-19 in Alexandria is almost certainly higher.

Virginia’s seven-day moving average COVID-19 positivity rate of 7% as of Nov. 16 was second highest behind North Carolina’s 8.1% among neighboring states and D.C. Maryland was next at 6.9%, followed by West Virginia at 6.1%** and D.C.’s 3.1%, while the nationwide rate stood at 10%. The City of Alexandria’s rate of 6% was second-lowest, behind that of D.C.

Trying to navigate the many variables among the confusing and sometimes conflicting COVID-19 data can prove daunting. CDC data based on average daily cases relative to population has Virginia doing extremely well compared to other states, even though the seven-day average positivity rate found on individual state websites reveals that Virginia’s rate is second-highest in the region.

The bottom line is that COVID-19 cases are up nationwide and the United States is clearly in the midst of a third surge of the pandemic. But by at least one crucial measure, Virginia is currently faring better than most of the rest of the nation.

* These dates were chosen to provide monthly comparison points. They do not necessarily represent the high or low data points for the positivity average.

** Seven-day data was not available on the West Virginia website dating from Nov. 16. This number represents a five-day average for West Virginia only.