How to understand COVID-19 case counts, positivity rates, and other numbers
COVID-19 dashboards can serve as crucial resources for those following the pandemic. These dashboards pull statistics into one place and visualize the disease's impact over time. As fall starts, nine months into the COVID-19 pandemic, every state in the country and many major research institutions, news outlets, and volunteer efforts have developed pages devoted to tracking the outbreak. Some colleges, universities, and even school districts are getting in on the game.
The issue with COVID-19 dashboards is that they are inherently saturated with numbers. From case counts to death tolls, these statistics are arranged in a dizzying array of colorful tabs. It can be difficult to know where to click or what to read, much less how to take in all the data presented and understand how the pandemic is currently unfolding in your community. COVID-19 data have also become intensely political, as theorists on both sides of the aisle use numbers to advance their agenda rather than diving into what, exactly, the numbers mean.
Stacker consulted common COVID-19 sources such as the Centers for Disease Control and Prevention (CDC), the COVID Tracking Project, and the Council of State and Territorial Epidemiologists (CSTE) to compile a list of terms, principles, and data sources that can help you understand key COVID-19 numbers. Each slide provides context and information on where these numbers come from.
Read on to learn how to interpret your local COVID-19 dashboard and what data questions to ask at the next town meeting.
Editor's note: Betsy Ladyzhets is a volunteer with the COVID Tracking Project at The Atlantic, data from which was used for this piece.
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A COVID-19 case refers to a person infected with the virus SARS-CoV-2 and is now sick with COVID-19. Cases may be symptomatic—meaning this individual exhibits common COVID-19 symptoms, such as a fever, cough, or shortness of breath—or asymptomatic, meaning they do not exhibit symptoms. It is important to include both types of cases in case counts to show where and how fast the virus is spreading.
[Pictured: In this photo illustration the World Health Organization (WHO) Director General Tedros Adhanom Ghebreyesus is seen with a WHO coronavirus cases map displayed.]
When a COVID-19 case is confirmed, this means that the infected individual has been diagnosed using an FDA-approved polymerase chain reaction (PCR) test. PCR tests work by searching for genetic material that matches the novel coronavirus in a sample of your mucus, saliva, or other material. These tests are highly accurate, which is why the Council of State and Territorial Epidemiologists (CSTE) considers them to be an effective test for identifying COVID-19 cases.
A probable COVID-19 case, on the other hand, is identified through symptoms, exposure, or other types of tests that are less accurate than PCR. You may have a fever, chills, and a sore throat; you may have recently gotten lunch with a friend who tested positive for COVID-19 after you hung out; or you may have tested positive with an antibody or antigen test. But with any (or all) of these options, you would still need to take—and test positive on—a PCR test to be counted as a confirmed case of COVID-19.
A suspected case of COVID-19 is similar to a probable case, in that an individual is presumptively diagnosed based on symptoms or exposure to the virus but has not had their disease confirmed through PCR testing. Some public health agencies use the term "suspected" instead of probable to refer to hospital patients who have COVID-19 symptoms, but have not yet been tested or are awaiting test results.
A count of cumulative cases refers to all the people in a given region or institution who have been sick with COVID-19; such a count demonstrates how hard COVID-19 has hit a particular region or population. For example, according to the New York City public health department, 233,216 people in New York City have had the virus as of Sept. 12. That amounts to about 2.7% of all New Yorkers.
While cumulative case numbers reflect how many people in a given population have contracted the novel coronavirus overall, daily case numbers reflect how the coronavirus is progressing over time. Many public health agencies report daily cases on COVID-19 dashboards or in press briefings; they are often demonstrated in a line chart that makes it easy to compare today's number to yesterday's, last week's, and last month's.
Seven-day average cases
The problem with using daily case counts as a metric to track COVID-19 is that they often fluctuate due to factors unrelated to COVID-19 itself. Many laboratories—which process COVID-19 tests—and public health departments—which aggregate and report the results of those tests—do not work on weekends. This tends to cause fewer cases to be reported every Saturday and Sunday, while more cases are reported on Mondays to make up for the lag. As an example, check out Michigan's dashboard (daily cases tab): you can see drops in the new case numbers every weekend, and spikes at the start of each week. Significant amounts of new cases may also be added.
To account for this fluctuation, many researchers and reporters calculate a seven-day rolling average—the average of a particular day's new cases with the six days that came prior. This averaging practice smooths out daily peaks and falls into a trendline, which better reflects how cases are progressing over time. As ProPublica's Caroline Chen puts it: "It's kind of like the stock market; it's unhelpful to obsess over daily swings."
Active cases refer to the individuals who both have been infected with the novel coronavirus and pose a risk to other people in their community. It's essentially the cumulative number of cases, minus those people who have either recovered from the disease, completed their time in quarantine, or passed away.
As many places in the U.S. do not have enough public health staff to keep track of how each COVID-positive person in their purview is doing, many states and counties cannot report active cases. However, this metric has become a popular figure on college and university COVID-19 dashboards, like this one from Carleton College, to let students know how many of their peers are currently sick and isolated on campus.
Transmission rate (R0)
R0, also called the transmission rate or reproduction number, is a figure calculated by epidemiologists to show how contagious an infectious disease is. The figure describes how a disease would spread in a kind-of vacuum if nobody in the population has been exposed to the disease yet, and there is no way to control its spread. If R0 is less than one, this means epidemiologists expect one person infected with the disease to spread it to less than one new person; if it is greater than one, each infected person will spread the disease to more than one new person. Scientists have yet to agree on an R0 figure for the novel coronavirus, as its progression through different populations is still under study.
Polymerase chain reaction (PCR) tests are the gold standard for diagnosing COVID-19. The tests are highly accurate because they identify whether genetic material matching the novel coronavirus is present in a person's body—akin to scanning your nose or throat for a super-specific barcode. PCR tests tell you whether you have the coronavirus right now, making them what scientists call diagnostic.
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